A few Global.env removed.
A few improvements on pattern-matching compilation.

- Optimize the removal of generalization when there is no dependency in
  the generalized variable (see postprocess_dependencies, and the removal
  of dependencies in the default type of impossible cases).
- Compute the onlydflt flag correctly (what allows automatic treatment
  of impossible cases even when there is no clause at all).
Check compatibility of types in change depending on whether it is a
term or a type.

Continuation of 9a82982c1eb.