Implement simple allocator using enif_{alloc,free}

[filmor]

Implement a Rust global allocator using Erlang's enif_alloc and enif_free.

Rumor has it that this makes Rust's memory usage show up in erlang:memory().

The tests are currently reproducibly seg-faulting on Linux x86-64 but run through on arm64.

[filmor]

TODO: Check with valgrind what's going on

[dvic]

Locally on my Mac M1 both the Rust and Elixir tests are passing fine, so it might be something specific to the CI?

[filmor]

Some notes:

• The error doesn't always happen on my machine (amd64, Linux). Currently, roughly every 5th full test suite execution segfaults at a random test.
• The seed doesn't matter. Even with a seed that failed before, the next execution can go through.
• I haven't had the time, yet, to run this with a debug Erlang instance.
• This is not a blocker for new releases.

[filmor]

Reading https://github.com/E-xyza/zigler/blob/main/priv/beam/allocator.zig brought the fix, this works now.

[filmor]

@evnu As requested, added more comments to the allocator.