When using `--external-tokens M`, Menhir assumes that the terminal GADT is
`M.MenhirInterpreter.terminal`.

However, the module generated using `--only-tokens` exposed `terminal` at the
top-level.  This lead to a different interface for M depending on whether it
was generated using `--only-tokens` (`M.terminal`) or as a normal parser
(`M.MenhirInterpreter.terminal`).

This patch fixes that case: `terminal` can be referred to uniformly using
`M.MenhirInterpreter.terminal`.