[Feature] no_alloc operation

[adryzz]

The way i understand it, this crate uses alloc for fancy error reporting (and operation on Strings, Vecs and Cows).

It would be nice to be able to use this crate (albeit with worse error reporting) in a no_std and no_alloc environment, of course without String/Vec/NullString types (or perhaps using heapless to provide stack-based alternatives).

I'll try doing this in a branch and see how much is given up by doing this.