format namespace use

Summary: The hack formatter currently only supports use statements of the form use namespace; This extends
it to support statements of the form use namespace as x;

Reviewed By: @jwatzman

Differential Revision: D1669866

Signature: t1:1669866:1415659133:563d3f046f41919df5444b156292b36475df674a
parse pass by reference and represent it in the AST

Summary: parameters that are passed by reference are marked as such.

Reviewed By: @jwatzman

Differential Revision: D1669930
adding support for assignment refs

Summary: Represents a reference of an expression in the ast and parses it.

Reviewed By: @jwatzman

Differential Revision: D1669949