Migrate Unicode Extension Keywords to ShortVec and improve macro

[sffc]

trafficstars

Forked from #1800 to cover Unicode extension keywords.

We should be able to write:

locale!("en-US-u-ca-buddhist")

[yzhang1994]

@sffc Quick question: similar to the variants, I believe there should be some limitations on what we can parse in a const context. Will the locale macro only support a single unicode key value pair? What about other parts of Extensions e.g. Transform, Private? Unicode Attributes?

[sffc]

Good question; I want to start with just unicode::Keywords because those are the most common requirement. The other items can be added later if they are needed.