Issue #24221: Small optimizations for heapq.

Replaces the PyList_GET_ITEM and PyList_SET_ITEM macros with normal array
accesses.  Replace the siftup unpredicatable branch with arithmetic.
Replace the rc == -1 tests with rc < 0.  Gives nicer looking assembly
with both Clang and GCC-4.9.  Also gives a small performance both for both.