Add optional support for compact_str::CompactString

[mcronce]

Think I got everything right (it's definitely working for my use case, at least, building against local bitcode) - I have one open question in a // TODO comment, but I suspect the answer is "no we can't" and also "it would hardly make a difference even if we could"

[finnbear]

Thanks! Next time we work on bitcode, we'll review and merge this. Regarding your TODO comment, I don't think it can be optimized. CompactString can be:

• 'static but, without lifetime specialization and decoding a 'static buffer, it cannot be exploited in bitcode
• inline but, here, a copy is unavoidable without changing the StrDecoder internals. if you give a &'str internals as if they were for a String, then it will cause a use-after-free when impl Drop for String runs.
• heap but, here, you must create an allocation where none previously existed

If CompactString had a borrowed variant, like enum CompactString<'a> { Borrowed(&'a str) } (or the branchless equivalent of this) then maybe it could borrow from the buffer.