`#![no_std]` support

[logannc]

The only standard library uses we currently have are HashSet, for which there are excellent replacements, and std::cmp::{min, max} which are obviously functions we can live without.

[seanpianka]

Should we mark this for 1.0?

[logannc]

I don't think it requires an API change since Vec is declared in alloc which is no_std compatible and I think the HashSet is an implementation detail for token_sort, etc.