In this version, I use modern types, modern style, simplify the use of Header,
and add the bare-minimum safety of arithmetic checking.
The actual significant change is that more_memory (we no longer use magnetic
core memory, so I changed the name of the function!) uses mmap, not sbrk.
This allocator still never returns memory back to the operating system (by
calling munmap), though.
This allocator does not incorporate any of the improvements suggested in the exercises in K&R, p. 189.
Most of the added lines in this version are comments that explain why the code is the way it is. Hopefully, the comments are sufficient and correct.