Lint theory names

This issue is to add checks on theory name conventions to the linter (implemented as pat of readme_gen). In particular, the following conventions should be followed for theory names:

• lowercase letters and underscores only
• the following suffixes as exceptions to the rule above
  • Proof [proofs about the corresponding suffix-less theory's definitions]
  • Lang [definition of an intermediate language]
  • Sem [semantics of an intermediate language]
  • Props [auxiliary properties and proofs for an intermediate language or compiler pass]
  • Prog [construction of a deeply embedded program and its verification]
  • Compile [in-logic compilation of the corresponding Prog theory's program]
  • CompileProof [machine-code level proof for the corresponding Prog theory]

Optionally, we could also check that whenever a Proof exists, a corresponding suffix-less theory exists, and similar checks for other requirements for suffixed theories, including their location in the directory tree.

(In future, this linting could be used to enforce a CakeML-wide theory suffix/prefix (to avoid clashes with other developments).)