Start implementing code generation

I'm able to compile and run test.scm, which successfully prints 3 to
stdout. The standard library is nowhere near complete, and in
particular, I want to be able to work with values other than integers.

I brought in petewarden's simple hashmap implementation for C because
I'm not interested in implementing a hashmap for the purposes of this
project.
Make project more developer-friendly

- Add README.md with Quick Start instructions.
- Allow compiling any input file.
- Add output files into .gitignore.