Allocation to use mmap

[skx]

This pull-request moves from using a null area in the BSS section (8Gb) for our memory allocation to using mmap to allocate an area of RAM at startup (16gb).

In order to make this transition I first created a general-purpose "allocate" routine and moved all code to use that. Then updated it to use mmap as the backing store, and added bound checking to it

The plus side is this works. The downside is that performance seems to be very variable:

$ time ./inception brainfuck.lisp --main
Loading .. brainfuck.lisp
Hello World!

real	1m13.950s
user	0m6.069s
sys	0m2.647s

$ time ./inception brainfuck.lisp --main
Loading .. brainfuck.lisp
Hello World!

real	2m42.343s
user	0m7.437s
sys	0m4.618s

$ time ./inception brainfuck.lisp --main
Loading .. brainfuck.lisp
Hello World!

real	1m22.966s
user	0m6.954s
sys	0m3.189s

Need to investigate before merging, and compare to main and/or the previous release.