Alias Flow_ast as Ast in flow_ast_mapper.ml
Alias Flow_ast as Ast in flow_ast_mapper.ml

Summary: Flow_ast is heavily used in the Flow AST mapper, and repeating the full name everywhere is verbose (and in some cases makes it hard to keep reasonable line lengths while being reasonably formatted). Alias it to just `Ast`, which is what `flow_polymorphic_ast_mapper.ml` uses.

Reviewed By: panagosg7

Differential Revision: D15638159

fbshipit-source-id: 8d23a3b0e99b6adfdd4263c3f2dbeaecff7370b3

samwgoldman/flow,gabelevi/flow,gabelevi/flow,facebook/flow,mroch/flow,facebook/flow,gabelevi/flow,facebook/flow,facebook/flow,gabelevi/flow,mroch/flow,gabelevi/flow,mroch/flow,facebook/flow,facebook/flow,samwgoldman/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,gabelevi/flow,samwgoldman/flow,samwgoldman/flow,gabelevi/flow,samwgoldman/flow