Thrift API unpack function

[eightysteele]

We've surfaced a new Thrift API in feature/thrift-api which includes an unpack function for unpacking any Thrift object. There's a few concerns about this.

Basically Thrift objects don't really have ordering by default, so what should you get back when unpacking? Consider an IDL struct like this:

struct TimeValue {
  1: i32 timestamp
  2: optional string title
  5: i64 value
}

We need to make sure unpack responds consistently for populated and un-populated fields. Also instead of hardcoding unpack implementations, we should reflect on the class to get field names. It should "just work" for any Thrift object.