Feature request: const fn new

[kstrafe]

Making Bump::new be const fn allows for use in thread locals via

thread_local! {
    static X: Bump = const { Bump::new() };
}

This makes thread local access much faster because we do not need to perform a dynamic check for each access to the thread local.

[bluurryy]

If we implemented new like bumpalo using their EMPTY_CHUNK trick, we could make it const in nightly with #![feature(const_refs_to_static)].

The thing is that we have a doubly linked list of chunks instead of singly linked list. This is so we can reuse and don't have to deallocate chunks that were allocated in a scope.

We would have to check every time we create a new scope or checkpoint if we are pointing to that EMPTY_CHUNK and if so allocate a real chunk that we can mutate and link up with future chunks.

EDIT: Is that tradeoff worth it?

[bluurryy]

Bump allocators can now be constructed without allocation via Bump::unallocated. This only works with the Global allocator.
It's const when the feature nightly-const-refs-to-static is enabled.

type Bump = bump_scope::Bump<Global, 1, true, false>;

thread_local! {
    static BUMP: Bump = const { Bump::unallocated() };
}

An unallocated bump allocator can't create scopes or checkpoints, but everything else works.
You can use scopes by converting it into a guaranteed allocated bump allocator with into_guaranteed_allocated or as_guaranteed_allocated(_mut).

EDIT: updated function names for v0.3