Implement a proper bump allocator

[blt]

This commit introduces a proper thread-safe bump allocator to bh_alloc. The previous allocator is now moved to the fuzz sub-module. This work was inspired by a Reddit conversation. I felt pretty bad that my poor fuzzy allocator got held up to demonstrate a general purpose bump allocator. :)

Anyway, the only dicey bit here is the AtomicUsize. I haven't done any stress testing on an ARM machine, which should be done. x86 has stricter memory ordering, hiding bugs that only turn up under ARM.

Signed-off-by: Brian L. Troutwine brian@troutwine.us

[SimonSapin]

Thank you for working on this!

I think there may be a bug in the alignment handling. It’s not the size that needs to be rounded up, is the addressed of the returned pointer. [u8; TOTAL_BYTES] has align 1, so HEAP could start at any address. For example if it starts at address 0x1_0003, offset is 0, and the requested alignment is 8, then the allocator should return a pointer with address 0x1_0010.

Although the size of Rust types is always a multiple of their alignment (to make arrays work), this is not necessarily the case of Layout. The end offset does not need to be aligned.

[blt]

@SimonSapin hey, thanks for this. I put this together without a test plan and fully believe there are bugs in it. Would you mind opening an issue?