Remove use of charconv (not supported well)
Remove use of charconv (not supported well)

* Remove FixNum, FixNat, FixHexNat, FixInt, FixHexInt, FixFloat
* Add ParseInteger template function for parsing decimal and hex
* Implement StrToNat with ParseInteger
* Implement StrToInt with StrToNat
* Use ParseInteger in StrToFloat instead of from_chars

WebAssembly/wasp,WebAssembly/wasp,WebAssembly/wasp