heap/linker/main: set up a heap allocator

heap/linker/main: set up a heap allocator

This commit sets up a heap allocator.  Again, I cannot sufficiently
stress the usefulness of Meyer Zinn for getting things this far, and
much of what I needed to know about getting linker symbols propagated
into Rust code comes from:
https://meyerzinn.tech/posts/2023/03/08/p1-printing-and-allocating/

Meyer steps through the process of setting up the linked list allocator.
However, there's a buddy list allocator that can be used as a drop-in
replacement, and since this is a more common allocator scheme, and since
doing my own thing means writing my own code and getting more familiar
with Rust, it seemed a divergence worth taking.