Avoid allocation in PathString.Add (#28855)

Also some other refactorings based on IDE's recommendations

```
|    Method |     Mean |    Error |   StdDev | Ratio |
|---------- |---------:|---------:|---------:|------:|
|  Baseline | 18.56 ns | 0.099 ns | 0.088 ns |  1.00 |
|    Create | 12.70 ns | 0.174 ns | 0.154 ns |  0.68 |
```

Author: pranavkm

src/Http/Http.Abstractions/src/PathString.cs

@@ -19,7 +19,7 @@ namespace Microsoft.AspNetCore.Http
         /// <summary>
         /// Represents the empty path. This field is read-only.
         /// </summary>
-        public static readonly PathString Empty = new PathString(string.Empty);
+        public static readonly PathString Empty = new(string.Empty);
 
         /// <summary>
         /// Initialize the path string with a given value. This value must be in unescaped format. Use
@@ -103,11 +103,7 @@ private static string ToEscapedUriComponent(string value, int i)
                     if (requiresEscaping)
                     {
                         // the current segment requires escape
-                        if (buffer == null)
-                        {
-                            buffer = new StringBuilder(value.Length * 3);
-                        }
-
+                        buffer ??= new StringBuilder(value.Length * 3);
                         buffer.Append(Uri.EscapeDataString(value.Substring(start, count)));
 
                         requiresEscaping = false;
@@ -131,11 +127,7 @@ private static string ToEscapedUriComponent(string value, int i)
                     if (!requiresEscaping)
                     {
                         // the current segment doesn't require escape
-                        if (buffer == null)
-                        {
-                            buffer = new StringBuilder(value.Length * 3);
-                        }
-
+                        buffer ??= new StringBuilder(value.Length * 3);
                         buffer.Append(value, start, count);
 
                         requiresEscaping = true;
@@ -156,10 +148,7 @@ private static string ToEscapedUriComponent(string value, int i)
             {
                 if (count > 0)
                 {
-                    if (buffer == null)
-                    {
-                        buffer = new StringBuilder(value.Length * 3);
-                    }
+                    buffer ??= new StringBuilder(value.Length * 3);
 
                     if (requiresEscaping)
                     {
@@ -259,7 +248,7 @@ public bool StartsWithSegments(PathString other, StringComparison comparisonType
             {
                 if (value1.Length == value2.Length || value1[value2.Length] == '/')
                 {
-                    remaining = new PathString(value1.Substring(value2.Length));
+                    remaining = new PathString(value1[value2.Length..]);
                     return true;
                 }
             }
@@ -298,7 +287,7 @@ public bool StartsWithSegments(PathString other, StringComparison comparisonType
                 if (value1.Length == value2.Length || value1[value2.Length] == '/')
                 {
                     matched = new PathString(value1.Substring(0, value2.Length));
-                    remaining = new PathString(value1.Substring(value2.Length));
+                    remaining = new PathString(value1[value2.Length..]);
                     return true;
                 }
             }
@@ -315,11 +304,12 @@ public PathString Add(PathString other)
         {
             if (HasValue &&
                 other.HasValue &&
-                Value![Value.Length - 1] == '/')
+                Value[^1] == '/')
             {
                 // If the path string has a trailing slash and the other string has a leading slash, we need
                 // to trim one of them.
-                return new PathString(Value + other.Value!.Substring(1));
+                var combined = string.Concat(Value.AsSpan(), other.Value.AsSpan(1));
+                return new PathString(combined);
             }
 
             return new PathString(Value + other.Value);
@@ -366,11 +356,11 @@ public bool Equals(PathString other, StringComparison comparisonType)
         /// <returns>True if both PathString values are equal</returns>
         public override bool Equals(object? obj)
         {
-            if (ReferenceEquals(null, obj))
+            if (obj is null)
             {
                 return !HasValue;
             }
-            return obj is PathString && Equals((PathString)obj);
+            return obj is PathString pathString && Equals(pathString);
         }
 
         /// <summary>
@@ -471,18 +461,16 @@ internal static PathString ConvertFromString(string? s)
     internal sealed class PathStringConverter : TypeConverter
     {
         public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
-            => sourceType == typeof(string) 
-            ? true 
-            : base.CanConvertFrom(context, sourceType);
+            => sourceType == typeof(string) || base.CanConvertFrom(context, sourceType);
 
         public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
-            => value is string 
-            ? PathString.ConvertFromString((string)value) 
+            => value is string @string
+            ? PathString.ConvertFromString(@string)
             : base.ConvertFrom(context, culture, value);
 
         public override object ConvertTo(ITypeDescriptorContext context,
            CultureInfo culture, object value, Type destinationType)
-            => destinationType == typeof(string) 
+            => destinationType == typeof(string)
             ? value.ToString() ?? string.Empty
             : base.ConvertTo(context, culture, value, destinationType);
     }