Generate new structs for messages.

[kaeluka]

There's one struct for oneway msgs and one struct for future-msgs
each.

This encore method in class Foo

def bar(x:int, y:string) {...}

will generate:

struct ___encore_Foo_bar_fut_msg
{
  encore_fut_msg_t msg;
  int64_t x;
  char* y;
};

struct ___encore_Foo_bar_oneway_msg
{
  encore_msg_oneway_t msg;
  int64_t x;
  char* y;
};