Unicode conversion helpers for Str

[smores56]

from Zulip thread

We currently have a few utility functions for conversion from bytes to UTF-8 strings and back, which just got slightly updated in this PR. As a means to properly support differently encoded command line arguments across different operating systems (needed for roc-lang/basic-cli#289), we want to provide conversion functions from List U8/List U16/List U32 to Str. Here are the planned signatures:

Str.from_utf8 : List U8 -> Result Str [BadUtf8 { err : InvalidUtf8, index : U64 }]
Str.from_utf8_lossy : List U8 -> Str
Str.from_utf16 : List U16 -> Result Str [BadUtf16 { err : InvalidUtf16, index : U64 }]
Str.from_utf16_lossy : List U16 -> Str
Str.from_utf32 : List U32 -> Result Str [BadUtf32 { err : InvalidUtf32, index : U64 }]
Str.from_utf32_lossy : List U32 -> Str

The *_lossy variants should replace invalid codepoints with the Unicode replacement character, "\uFFFD".

[shua]

I'm working on this