Add a simple build script to bootstrap a build

This script works as instructions to a new developer as well as just
making everything a bit easier to trigger from editors, etc.

Yeah, the farce of having a build script to invoke a build script to
generate and invoke a buildscript to invoke a build is not lost on me.
I guess that's just part of doing C; generations of tools getting
slightly better at each time...
Rename parsing to parser
Start working on a parsing facade
Improve parser functions
Accept numbers as valid input
Support decimal numbers

Parser got quite a bit more complicated because I wanted "--1" to be a
parse error to avoid ambiguity. I wanted to force whitespace between an
operator and the numbers.
Hurr durr
Start working on a stupid evaler