bpo-36854: gcmodule.c gets its state from tstate (GH-17285)

* Add GCState type for readability
* gcmodule.c now gets its gcstate from tstate
* _PyGC_DumpShutdownStats() now expects tstate rather than runtime
* Rename "state" to "gcstate" for readability: to avoid confusion
  between "state" and "tstate" for example.
* collect() now only expects tstate: it gets gcstate from tstate.
* Pass tstate to _PyErr_xxx() functions