Investigate (optionally) allocating initial chunks in a bump arena on the stack

[fitzgen]

We could use const generics to control the size, perhaps.

This means that the initial N bytes of allocation wouldn't need to hit the global allocator.

cc @cfallin

[i404788]

Is this something that is still under consideration?

It would quite cool for embedded, since you'd be able to do 'dynamic' allocations on the stack without the need for a full allocator.

[fitzgen]

It would be a breaking change, at least as initially formulated, so won't happen until the Allocator trait is stable in std at the very least.