Initial Unicode module

[milseman]

This includes a port of UnicodeHelpers.swift from the standard library, and stubs for other stuff.

[atrick]

I don't get why a public decode function would take anything other than a raw pointer. The type to decode is explicit in the signature. A typed pointer is never the right way to view a block of memory that's about to be decoded.

If this is about wanting to internally work with a Collection of CodeUnit, so subscripting does the scaling-by-stride for you, then I'd use a trivial UnicideCodeUnit wrapper over the raw pointer.

[karwa]

We need Unicode functionality without allocating a String as an intermediary
...
We can't be bothered to consistently contribute to the stdlib because it's so painful

Preach it! 🙌

[milseman]

@atrick good point, we should change that (or have both overloads). As you know, there is no contiguousRawStorageIfAvailable equivalent, so types may live on for a while.

We’d want to migrate to a move only safe view as soon as that’s available. If anything, this module could be a prime example of why that’s so vital.

[milseman]

@swift-ci please test linux platform

[milseman]

Merging to unblock some lexer work.