identify enums as TType.ENUM

RB_ID=156181
show the filename of the file being parsed in error messages

RB_ID=156220
Create the ability to map includes to directories

RB_ID=156230
add ability to pass through additional fields

This enables one to create a proxy that may not necessarily be aware of
new structs coming through its pipeline making updating IDLs and
deploying changing much simpler.

RB_ID=156114
bump util/finagle deps
update changelog
bump version to 3.1.9