[ast-differ] clean up unnecessary Ast.Statement module prefixes
[ast-differ] clean up unnecessary Ast.Statement module prefixes

Summary: Match cases in `statement` fn were hard to read, so I cleaned them up.

Reviewed By: nmote

Differential Revision: D12826935

fbshipit-source-id: a2134298b14ce72860c7c70a2feeeef6626978b1

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow