Split arrow_flag into Parsetree.arrow_flag and Types.arrow_flag
Remove Typedtree.optional, as its purpose was redundant.
WIP: refining arrow_flag with apply_flag
Introduce "implicit" keyword and constructions in parser
WIP introducing implicit in typing
Make ocaml compile, add TODOs for implicit in typer
Change representation of implicits in AST
Introduce implicits in Typedtree
WIP: toplevel is working
Fix tools, leave FIXMEs in ocamldoc
Add Implicit flag to arrow_flag
WIP, generalizing arrows
WIP2