[parser] Factor out some common code in class_declaration and class_expression
[parser] Factor out some common code in class_declaration and class_expression

Summary:
Previously, the `_class` function returned a tuple, which was destructured in both class_{declaration,expression}, which then both created the Class.t record.

Factor our the common code. `_class` now returns a `Class.t`, which the other two functions wrap appropriately.

Reviewed By: panagosg7

Differential Revision: D18499013

fbshipit-source-id: cf2efc5dd94e131ee1dc66078e9ee16bc05ae450

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