Use sizeof() to get platform dependent struct size

This is only used in uv_fs for now. Will add everywhere once
the lifecycle stuff is finished.

Rename consts_stub.c functions so as to not collide with
macro names.
Lifecycle code working and ready to go with FS

This correctly stores both the callback, and data allocated
by OCaml. Ditched refcounts (because we aren't counting anymore)
in favor of a straightforward 'store unique id with the object'
module we're calling 'coat check'.

* This is just implemented in FS right now. Will roll out to the
  rest of the stuff once we implement the error handling.
* Minor cleanup in test_lifecycle