Simplify js-string conversion from primitive numeric methods

[eqrion]

From the overview:

String (extensions to the existing wasm:js-string): 

Conversion from primitive numeric types: fromI32, fromU32, fromI64, fromU64, fromF64

Would it be sufficient to reduce this to:

js-value:from: (x: T) -> externref
  generic over any value type `T`
  equivalent to ToWebAssemblyValue(ToJSValue(x, T), 'externref')

js-number:toString: (x: externref) -> externref
  if (typeof x !== "number") trap();
  return x.toString();

The js-value:from method would also be useful for #20.

[sjrd]

That does not work for fromI64 and fromU64. Those would need to go through bigints with such an interface.

For the other three, if that's the direction we're going, we would be better served with fromF64: (x: f64) -> externref. js-value:from would only be a complicated indirection. Engines could recognize the sequence

f64.convert_i32_s ;; resp. _u
call $stringFrom64

as actually a proxy for fromI32 (resp. fromU32).