WIP. Trying to clean up code generator code.
WIP. Trying to improve parser error messages.
WIP. Stupid lisp stuff.
Simplify and finish code generator

Simplify the code in the code generator a bunch and add
const-declaration, while-statement, and assignment to it, to finish it.
Merge branch 'dev' of https://github.com/butchler/PL0-compiler into merge

Conflicts:
	src/parser.c
	src/parser.h
	test/test.c