Changes

matching: matcher_* take Simple.pattern arguments
 (commit: 67c56e6)

matching: specialize_matrix uses non-empty rows
 (commit: 97caf28)

matching: consolidate all matcher_ functions in a single matcher_head

This commit is delicate and needs a careful review.

The `matcher_of_pattern` function is a temporary measure to reduce the
invasiveness of the patch, and make it easier to review.

(Note for reviewers: in the previous version the Record case had
a funny handling of Any, but it is in fact equivalent to just adding
omegas as we now do in all cases.)

There are two obvious directions for improvement:

- Get rid of matcher_of_pattern and pass a head directly to the
  various make_matching_* functions.

- Try to factorize this code with ctx_matcher which, it is now
  obvious, does essentially the same thing.

Another, less immediate area of attack would be to consider
a presentation of Pattern_head.t where the Any case can be statically
ruled out -- maybe the description could have two levels, one
isomorphic to option (Any or not?) and one for non-any heads.
 (commit: 0f5a1c4)

matching: refine the types in Context.ctx_matcher
 (commit: 387955e)

matching: finally, merge matcher and Context.ctx_matcher
 (commit: a26e509)

Changes
 (commit: ae4621c)

matching: use heads in the make_*_matching specialization calls
 (commit: ac1243c)

[minor] matching.ml: tune ~scopes handling
 (commit: 50fdc06)

matching: add a comment suggested by Florian Angeletti's review
 (commit: 3527653)

parmatch: normalize_pat is not used anymore
 (commit: 3903f9f)