Support no_std builds

[mbrubeck]

This adds two features, both enabled by default:

• normalization - enables the canonical_caseless_match and
  compatibility_caseless_match methods, which require the
  unicode_normalization crate.

• alloc - enables the default_case_fold_str convenience method,
  which requires allocating a String.

Disabling both these features allows use of this crate for case folding in
no_std programs.

[mbrubeck]

r? @SimonSapin

Note: The alloc feature could eventually depend on liballoc instead of libstd, if that subset is stabilized.

[CAD97]

I don't think you can cfg individual fn off of a trait where that trait is implementable outside of the current crate. cfgs are supposed to be purely additive, and a codependency activating one of these cfgs could break an unrelated crate that built without the feature.

[mbrubeck]

I don't think you can cfg individual fn off of a trait where that trait is implementable outside of the current crate.

Hmm, good point. We could make the trait private when the feature is disabled, and only provide the top-level functions, or we could make a breaking change and split it into two traits.

[mbrubeck]

We could make the trait private when the feature is disabled, and only provide the top-level functions

I implemented this for now, as it is simple and backward-compatible.

This reduces the API for no_std crates to just default_caseless_match_str. We could expand on that in the future.