Support no-std + liballoc environments

[bitwalker]

Hi!

I was interested in building a new parser for a language I work on, using rowan, but our tooling needs to support both std and no-std + liballoc configurations, and currently rowan does not support the latter.

Looking through the sources though, it doesn't appear like there is any actual requirement on libstd per se - all the references to things in the std namespace are things that are provided via core or alloc as well, unless I missed something somewhere.

Would a PR to make rowan no-std compatible (with a requirement on liballoc in that case) be accepted? I'm happy to do the work, just want to make sure there isn't some specific reason why it isn't supported already.

[lnicola]

No particular reason, I think. But note that rowan is currently pending a big rewrite, and its maintenance status is somewhat spotty.