Recycle - recycle owned values to avoid allocating

Status: just a prototype to benchmark the idea

In places where allocating is unavoidable. eg. when having to return String or Vec<u8> from a function, return Recycle<String> or Recycle<Vec<u8>> and reuse the allocated values using thread-local pools.

test alloc_write_vec         … bench:          74 ns/iter (+/- 4)
test alloc_write_recycle_vec … bench:          46 ns/iter (+/- 4)

test alloc_vec_u8            … bench:          22 ns/iter (+/- 1)
test recycle_vec_u8          … bench:          14 ns/iter (+/- 1)

test alloc_vec_u64           … bench:          23 ns/iter (+/- 1)
test recycle_vec_u64         … bench:          14 ns/iter (+/- 1)

I've found other library addressing same problem: https://github.com/frankmcsherry/recycler , but I think this syntax is much simpler to use.

Recycle<T> implements Deref and DerefMut, and could potentially be implemented for multiple more types, and interact well with Into AsRef etc.