Miscellaneous features and fixes to implement

[LPTK]

Syntax

• Special-case {} to mean an empty object (currently, one has to use new Object)
• Parsing this currently fails:

  foo(1,
    2)

Mutability/Immutability

• Make arrays and object literals immutable by default (using Object.freeze)
• Allow mut modifier on array and object literals
  This will remove the {} footgun (it resolves to runtime.Unit, which is currently not frozen!). For a fresh mutable records, users will need to write mut {}.
• Make class ctors use Object.freeze by default – this will probably require generating getters/setters for mut vals

Bugs

• Currently, && and || are not short-cirtuiting!

Code-gen improvements

• Should avoid generating very repetitive "else throw match error" structures

  Example

  data class Foo(a)
  
  :sjs
  case Foo(Foo(Foo(x))) then x
  //│ JS (unsanitized):
  //│ let lambda;
  //│ lambda = (undefined, function (caseScrut) {
  //│   let param0, param01, param02, x;
  //│   if (caseScrut instanceof Foo1.class) {
  //│     param0 = caseScrut.a;
  //│     if (param0 instanceof Foo1.class) {
  //│       param01 = param0.a;
  //│       if (param01 instanceof Foo1.class) {
  //│         param02 = param01.a;
  //│         x = param02;
  //│         return x
  //│       } else {
  //│         throw new globalThis.Error("match error");
  //│       }
  //│     } else {
  //│       throw new globalThis.Error("match error");
  //│     }
  //│   } else {
  //│     throw new globalThis.Error("match error");
  //│   }
  //│ });
  //│ lambda
  //│ = [function]