Add wasm-encoder API to directly encode instructions (#1985)

* Delegate `Instruction` encoding to a new type

* Refactor to prefer `InstructionSink`

* Remove the typed indices

* Fix build

Author: samestep

crates/wasm-encoder/README.md

@@ -26,7 +26,7 @@ And then you can encode WebAssembly binaries via:
 
 ```rust
 use wasm_encoder::{
-    CodeSection, ExportKind, ExportSection, Function, FunctionSection, Instruction,
+    CodeSection, ExportKind, ExportSection, Function, FunctionSection,
     Module, TypeSection, ValType,
 };
 
@@ -54,10 +54,11 @@ module.section(&exports);
 let mut codes = CodeSection::new();
 let locals = vec![];
 let mut f = Function::new(locals);
-f.instruction(&Instruction::LocalGet(0));
-f.instruction(&Instruction::LocalGet(1));
-f.instruction(&Instruction::I32Add);
-f.instruction(&Instruction::End);
+f.instructions()
+    .local_get(0)
+    .local_get(1)
+    .i32_add()
+    .end();
 codes.function(&f);
 module.section(&codes);
 

crates/wasm-encoder/src/core.rs

@@ -8,6 +8,7 @@ mod exports;
 mod functions;
 mod globals;
 mod imports;
+mod instructions;
 mod linking;
 mod memories;
 mod names;
@@ -27,6 +28,7 @@ pub use exports::*;
 pub use functions::*;
 pub use globals::*;
 pub use imports::*;
+pub use instructions::*;
 pub use linking::*;
 pub use memories::*;
 pub use names::*;

crates/wasm-encoder/src/core/branch_hints.rs

@@ -25,11 +25,12 @@ use alloc::vec::Vec;
 /// let mut code = CodeSection::new();
 /// let mut body = Function::new([]);
 ///
-/// body.instruction(&Instruction::I32Const(1));
+/// body.instructions().i32_const(1);
 /// let if_offset = body.byte_len();
-/// body.instruction(&Instruction::If(BlockType::Empty));
-/// body.instruction(&Instruction::End);
-/// body.instruction(&Instruction::End);
+/// body.instructions()
+///     .if_(BlockType::Empty)
+///     .end()
+///     .end();
 /// code.function(&body);
 ///
 /// let mut hints = BranchHints::new();