Introduce truly separate (sub)interpreter objects.  For now, these
must be used by separate threads, created from C.  See Demo/pysvr for
an example of how to use this.  This also rationalizes Python's
initialization and finalization behavior:

Py_Initialize() -- initialize the whole interpreter
Py_Finalize() -- finalize the whole interpreter

tstate = Py_NewInterpreter() -- create a new (sub)interpreter
Py_EndInterpreter(tstate) -- delete a new (sub)interpreter

There are also new interfaces relating to threads and the interpreter
lock, which can be used to create new threads, and sometimes have to
be used to manipulate the interpreter lock when creating or deleting
sub-interpreters.  These are only defined when WITH_THREAD is defined:

PyEval_AcquireLock() -- acquire the interpreter lock
PyEval_ReleaseLock() -- release the interpreter lock

PyEval_AcquireThread(tstate) -- acquire the lock and make the thread current
PyEval_ReleaseThread(tstate) -- release the lock and make NULL current

Other administrative changes:

- The header file bltinmodule.h is deleted.

- The init functions for Import, Sys and Builtin are now internal and
  declared in pythonrun.h.

- Py_Setup() and Py_Cleanup() are no longer declared.

- The interpreter state and thread state structures are now linked
  together in a chain (the chain of interpreters is a static variable
  in pythonrun.c).

- Some members of the interpreter and thread structures have new,
  shorter, more consistent, names.

- Added declarations for _PyImport_{Find,Fixup}Extension() to import.h.