Add an "allocator_api" feature

[TDecking]

This should cross out the last item in the wishlist (#183).

If the feature is enabled, the SmallVec type gets another type parameter for the allocator and makes use of the nightly Rust "allocator_api" feature. Additional allocator-based functions also become available, including the non-standard from_slice_in
that mimics the to_vec_in function in Rust.

[TDecking]

Was there ever a vision articulated for this feature? While I believe that the feature is supposed to behave as it does is in this PR, it also mandates a couple of compromises.

[mbrubeck]

Sorry for the delay in reviewing this. Just wanted to let you know this is still on my to-do list.

[dflemstr]

Hey, I would love to see this merged, and additionally make it possible to use on stable by for example using this crate as a poly-fill if we're not on nightly: https://crates.io/crates/allocator-api2