Parse IDL without sequential dependency

[ethe]

Make parser does not rely on sequential definition, like:

service PingPong {
    Foo echo(1:Foo param)
}

struct Foo {
    1: optional Bar test,
    2: optional SomeInt some_int,
}

struct Bar {
    1: optional Foo test,
}

const SomeInt SOME_INT = [1, 2, 3]

typedef list<i32> SomeInt

It was not be supported in Thriftpy before, cause Thriftpy uses a one-pass parser build by Ply, and it is relies on sequence obviously. I can not find a way to solve this problem with Ply, instead of remove it totally, so I just add another step after Ply analysis to fill incomplete type definition.

[sicklydove]

Thanks for fixing this! I'm no longer at the same company I was at when I raised Thriftpy/thriftpy#280, but I've passed it along to a former colleague to check out :)