Implement translation to C

This includes:

- [ ] Adding statements, ... to the C AST (defined in `Tip.Backend.AbstractC`)
- [ ] Implementing pretty-printing with proper indentation for the C AST
- [ ] Implementing the translation to C
- for simplicity, strict evaluation could first be used, until a proper implementation of thunks/graph reduction is in place for lazy evaluation

Start with the C AST defined in Tip.Backend.AbstractC and map the requested statements and other constructs it must represent. Implement and verify properly indented pretty-printing and translation to C, initially using strict evaluation if needed; done means all listed AST, printing, and translation work is complete.

Written by the indexing model from the issue text.