- Eliminate usage of 'builtin module' in description. This is not essential to the proposal and
    causes confusion around a similarly named JS proposal, which had different goals.
  - Clarify some minor points.
  - Make JS-API changes to WebIDL comprehensive.
  - Reword feature detection section to actually propose change to WebAssembly.validate method

  - Function builtin behaviors is defined using 'create a host function'
  - Clarify behavior around monkey patching using standard language
  - Clarify edge cases around nullability
  - Clarify edge cases around unsigned/signed integers
  - Restrict 'substring' behavior to normal cases
  - Use wasm helpers for when wasm instructions are needed

The existing WTF-8 operation in this proposal violated one of the goals of the
proposal: "don't create substantial new functionality" by introducing WTF-8
transcoding support to the web platform without prior precedent. The WTF-8
operation is removed because of this.

The naming for WTF-16 operations is reworked to refer to 'charCodes' instead
as that is what the JS String interface uses.

We could support UTF-8 transcoding by referring to the TextEncoder/TextDecoder
interfaces, so this commit adds support for that.