Move Type_sig_merge into typing target
Move Type_sig_merge into typing target

Summary:
This module converts type signatures into signature contexts. Under types-first,
we create signature contexts from sig ASTs by passing those sig ASTs through the
existing codepaths in Statement, Type_annotation, Flow_js, etc. Under
types-first 2.0, we instead convert directly from type sigs to types, which is
the responsibility of the Type_sig_merge module.

Reviewed By: panagosg7

Differential Revision: D22875639

fbshipit-source-id: 8553fa048598cef6857b0d876f1bc07a3da08200

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