[WIP] Simply typed λ calculus

- [ ] named terms
- [ ] production of nameless terms from named terms
- [ ] translation of De Bruijn indices back into names
- [x] evaluation of terms
  - [x] shifting
  - [x] substitution
- [ ] parsing
  - [ ] use the parser from the λ target
  - [ ] change the rule for application to require parentheses so as to avoid left recursion
- [ ] repl

Alright, here goes.