Add interner lifetime and improve documentation

[Caellian]

This commit adds 'i lifetime that can be used by backends to return &'i str when it's known that the backing string storage won't be moved.

This is the case for fixed capacity pool-like backends and backends which avoid backing buffer moves via double indirection.

StringInterner::resolve can now return &'i str for backends that support it, or a local (call-site bound) reference. The local reference prevents insertions into StringInterner due to XOR mutability, while &'i str doesn't.

This is related to #75.

TODO

• Add 'i lifetime
• Document behavior described in Document interned string location #75
• Improve BucketBackend so it's capable of resolving to &'i str.

[Caellian]

New comparison table looks like:

light

dark

ayu

I've also described how each backend actually works - until now I believed StringBackend and BufferBackend are identical except BufferBackend uses Vec<u8>, I think this illustrates well that these details were missing.

[Caellian]

Something I didn't foresee while creating this PR is that BucketBackend is effectively a backend which doesn't have to move its contents. It's currently implemented using Strings which prevent it from handling that properly so I'll improve it as well in this PR just so it doesn't merely break the API with no benefits.