Fix several linting issues related to the new linter

[Maelan]

Each of the two commits fixes one the following issues (which were both noted as FIXMEs in the test file).

1. exception E = F : the equal sign was wrongly shown as an error; the fix is easy. This addresses a portion of spurious error on module type #84.
2. fun ... : t -> e : the type context was wrongly running over -> ; the fix introduces a little extra bit of complexity (fun is now the start of a region rather than just a keyword) but it is simpler than I anticipated (thanks to the containedin= feature of Vim). This closes coloring is wrong in some contexts #86.

Remaining issues:

• the test file has one remaining FIXME (but of much lesser importance, it is more a feature wish than an actual issue): class ['a] myclass ... where we’d like the type variable to be highlighted as such;
• spurious error on module type #84 has one more issue: (module M) used as a type expression.

[copy]

Thank you!