FixedStorage / StorageResizable

Hi! Someone on reddit [mentioned your storages-api](https://www.reddit.com/r/rust/comments/viiw79/comment/idek6xv/?utm_source=share&utm_medium=web2x&context=3).

I made similar storage/mem interface for my [any_vec lib](https://docs.rs/any_vec/0.9.0/any_vec/index.html#membuilder). I called it MemBuilder. MemBuilder constructs Mem, Mem then can be used as your Storage.

The thing I wanted to point out, is that I found it useful to split Mem on Mem + MemResizable and MemBuilder on MemBuilder + MemBuilderResizable. That allowed my Vec implementation to opt-out size-change operations whenever provided Mem implementation does not support them. For example, `reserve_exact` have no meaning for Stack allocated memory (InlineStorage of yours), and [thanks to the type knowledge we can opt out it compile time](https://docs.rs/any_vec/0.9.0/src/any_vec/any_vec.rs.html#197-201), without making separate implementation for it. Same thing with [with_capacity](https://docs.rs/any_vec/0.9.0/any_vec/struct.AnyVec.html#method.with_capacity).

In your api you could move out `grow` and `shrink` into some `StorageResizable`. That would allow `storage-api` consumers to opt-out operations compile time in their container implementations. Or at least make marker trait for that.

Just wanted to share my "experience" from similar design....