Add multi-value support

[abelstuker]

Closes #297.

This extends the implementation to support WebAssembly's multi-value proposal, allowing functions and control structures to return multiple values. This change aligns with the accepted proposal https://github.com/WebAssembly/multi-value/blob/master/proposals/multi-value/Overview.md.

Functions can now return multiple values:

;; Function returning two values
(func $add-and-subtract (param i32 i32) (result i32 i32)
  (local.get 0) 
  (local.get 1)
  (i32.add)
  (local.get 0) 
  (local.get 1)
  (i32.sub)
)

;; The function call returns two values
(i32.const 10)
(i32.const 5)
(call $add-and-subtract)
;; Stack now has: 15, 5

Control structures support multi-value types:

;; Block with multi-value result
(block (result i32 i32)
  (i32.const 42)
  (i32.const 99)
)

;; If/else with multi-value result
(if (result i32 i32)
  (then
    (i32.const 100)
    (i32.const 200)
  )
  (else
    (i32.const 300)
    (i32.const 400)
  )
)

The host can now invoke functions with multiple return values:

// Invoke function and capture multiple results
StackValue outputs[8];
uint32_t out_count = 0;
bool ok = this->invoke(m, fidx, 0, nullptr, 8, outputs, &out_count);

// Access individual results
int32_t result1 = outputs[0].value.uint32;
int32_t result2 = outputs[1].value.uint32;

[tolauwae]

@carllocos Multivalues should work now in WebAssembly.

Important caveat: support for it in other languages is still missing. Both Rust and AssemblyScript do not support it as far as I can tell. This does present a problem for having multivalue primitives.

[tolauwae]

@carllocos Multivalues should work now in WebAssembly.

Important caveat: support for it in other languages is still missing. Both Rust and AssemblyScript do not support it as far as I can tell. This does present a problem for having multivalue primitives.

I am partially wrong. Wasmtime docs still show how it works with tuple returns in Rust (https://docs.wasmtime.dev/examples-multi-value.html) but Rust itself does not output should functions anymore since a change in LLVM19 back in 2024.

[tolauwae]

Great work 👍