WIP - add senum to thrift-parser

[ugodiggi]

A senum is like an enum but does not have the int identifier - so it is just a string when serialized.

e.g.

senum Foo {
  VAL0,
  VAL1
}

See more details on the rationale here: https://docs.google.com/document/d/1VuKLrMyYMhEOTCaovQQii2wd0bUhp53FcqAYUeoS-QE/edit