refactor organisation of bootstrap theories

- move them all into subdirectories of bootstrap
- organise the subdirectories in three categories:
    translation, evaluation, and proofs
- move some other theorems around so that bootstrapping depends less
  on the compiler and repl correctness proofs (it still depends on
  compiler proofs for compute_free_varsLib - this can be fixed later)
- rename theories somewhat more consistently
- add/fix/update READMEs

(the main motivation was to put the compiler translation
 into a separate directory)