feat(alloc): impl GlobalAlloc for buddy::Alloc (#95)

this PR adds an impl of `GlobalAlloc` for `mycelium_alloc::buddy::Alloc`
so that we can reuse the buddy allocator as a heap allocator. no
functional change here.

in order to add this, i refactored the allocation code out of the
`page::Alloc` impl into inherent methods that operate on `Layout`s and
addrs, and reimplemented page allocation on top of that.

since we now operate on `Layout`s internally, we also rely on some of
their additional invariants.

note that i haven't actually tested this, but, like...it probably works?