Changes

patterns: move Parmatch.Pattern_head into Patterns.Head

The aim is to also move the Simple/Half_simple/General stuff from
matching, but we need to split in those modules the part that are
purely structural (they go in Patterns) and the parts that are
actually compilation logic (Half_simple.of_clause), those stay in
Matching.
 (commit: ffb6cae)

make depend
 (commit: e880ec3)

[minor] matching: rename Non_empty_clause.{map_head => map_first}
 (commit: 54b79d3)

matching: separate types for rows and clauses

"rows" are common abstraction of both pattern analysis and
compilation, but clauses carrying a Lambda.t term are
compilation-specific. Separating rows will thus enable moving more
logic to typing/patterns for use in both settings.
 (commit: f5f4ba6)

patterns: reuse ('a Typedtree.pattern_) to define Patterns.Head.t
 (commit: 0e979b7)

matching: move (Non_empty_row, views, General) to patterns.ml
 (commit: 4d6267d)

matching: move {general,simple,half_simple}_view to Patterns
 (commit: e19a3af)

matching: use pattern views in Parmatch as well
 (commit: 68dc87c)

Changes
 (commit: 229df94)

make check-typo happy
 (commit: 9ca2a35)

fixup! matching: use pattern views in Parmatch as well
 (commit: 1ee6ee4)