Can't use with `hashbrown/nightly` feature

[Imberflur]

This crates uses allocator-api2 without exposing a feature to enable #![feature(allocator_api)].

Due to issues outlined here zakarumych/allocator-api2#19,
this makes it is impossible to have this crate anywhere in the dependency tree along with hashbrown when enabling the hashbrown/nightly feature.

[cfallin]

PR welcome -- I'm not really that familiar with the present allocator API situation. Adding a Cargo feature for this sounds reasonable to me.

It looks like our use of the crate came in with #196 -- @jakubDoka , any thoughts?