Initial import of existing codebase.

So far all it can do is compile and execute a hard-coded statement,
but the code to build an AST and compile it into a program is there.
There's a lot of missing syntax checking/error handling because this
is a WIP and also because I'm new to python.

Also, substatements are implemented in the Lexer but not the
Parser, so if you try to use them it's gonna be awkward.