The jane branch was previously about 7 months behind upstream, so this rebase catches us up. Rebases were getting unwieldy due to the divergence of upstream and poor commit history of jane, so squashing these commits into one made it easier to resolve conflicts, especially in places where fixes to earlier merge conflicts depended on changes introduced in later commits.

Notable changes in this commit:

Initial patches to parser for janestreet specific extensions (local_, global_)
Support for include functor
Support for polymorphic function parameters
Support for comprehensions
Support for immutable arrays
Support for layout annotations
Support for exclave_ syntax
Support for unboxed float literals and float#
Support for explicit strengthening
Mode for rewriting local-mode attributes into their keyword form
Changes authored by:

antalsz
ccasin
cwong-ocaml
goldfirere
Julow
riaqn
rleshchinskiy
stedolan
tdelvecchio-jsc