The command line parser accepts any flags in `generate`, but no
warning is shown if user instructions are ignored.

This is a best-effort way of warning if a flag would be ignored,
making it more user-friendly.

Ideally, this would be done before starting work, but I don’t see a
good way to patch it into the command line parser; maybe by having an
intermediate parsing step that parses into an ADT first and throws
warnings for ignored arguments.

Another case I saw is that “--haskell” and “--frege” can both be set,
but “--haskell” will always be ignored.