Deprecate senum

I propose the senum type be deprecated (flagged by the compiler as scheduled for removal) in v1.0 and subsequently removed in a later version of Apache Thrift.

Rationale: The senum type is a string in all target languages. Instances of the type are not constrained to the enumeration strings in IDL or output languages. IDL constants of type senum can be assigned irrational values (e.g. random strings, integers, floats, etc.) without a compiler warning or error. The senum type offers no cross language utility and is easily replaced by string or enum (which offers string lookup in many languages). This type impacts a wide range of concerns including language generators and the effort required to learn Apache Thrift.