Support JS String Builtins

[oovm]

I hope jco transpile can implement JS String Builtins Proposal.

This helps to significantly reduce the size of browsers by reducing string-related functions and constant conversions.

---

The specific logic is to determine whether (import "wasm:js-string" ...) exists in core module.

If it exists, then add builtins: ['js-string'] to instantiateCore function.

---

Another logic is to add the --imported-string-constants option.

When --imported-string-constants STATIC_STRING is entered, the importedStringConstants: "STATIC_STRING" parameter will be added to the instantiateCore function

[guybedford]

We could easily add this to Jco, as you have described. Will keep this open to track. Are there toolchains that would be able to utilize this feature already?

[oovm]

moonbit supports this: https://www.moonbitlang.com/blog/js-string-builtins

This is the artificial:

;; jco transpile palindrome.wat --imported-string-constants "_"
(core module
  (type (;0;) (func (param externref) (result i32)))
  (type (;1;) (func (param externref i32) (result i32)))
  (type (;2;) (func (param (ref extern)) (result (ref extern))))
  (type (;3;) (func (param i32)))
  (type (;4;) (func))
  (import "wasm:js-string" "length" (func (;0;) (type 0)))
  (import "wasm:js-string" "charCodeAt" (func (;1;) (type 1)))
  (import "_" "true" (global (;0;) (ref extern)))
  (import "_" "false" (global (;1;) (ref extern)))
  (memory (;0;) 1)
  (export "palindrome" (func 2))
  (start 3)
  (func (;2;) (type 2) (param (ref extern)) (result (ref extern))
    (local i32 i32 i32)
    local.get 0
    call 0
    local.tee 1
    i32.const 2
    i32.div_s
    local.set 2
    i32.const 0
    loop (type 3) (param i32) ;; label = @1
      local.tee 3
      local.get 2
      i32.lt_s
      if ;; label = @2
        local.get 0
        local.get 3
        call 1
        local.get 0
        local.get 1
        local.get 3
        i32.sub
        i32.const 1
        i32.sub
        call 1
        i32.ne
        if ;; label = @3
          global.get 1
          return
        end
        local.get 3
        i32.const 1
        i32.add
        br 1 (;@1;)
      end
    end
    global.get 0
  )
  (func (;3;) (type 4))
)

[guybedford]

Note you could also experiment with this today using Jco's custom instantiation mode. I think one of the hardest questions will just be what string builtin import name to use.