use mmap over brk
Created by: chpio
Use mmap instead of brk for system memory allocations.
advantages:
- allows us to return allocated memory more "freely" to the system
- mmap is threadsafe
- allocate protected memory (non-exec, exec but not write)
- sbrk/brk is deprecated on MacOS #41
- [a wrapper/shim/polyfill would allow us to use this on brk-only systems]
- ?
disadvantages:
- the memory pool isn't contiguous anymore (is this even a problem for ralloc?)
- ?