Makes `import functools` about 1.5 ms faster.

* heapq is used only when Counter.most_common(n).
* types and waakref is used only for singledispatch.

master:

import time:   - _functools 76 us (self 76 us)
import time:       - _operator 110 us (self 110 us)
import time:     - operator 469 us (self 359 us)
import time:     - keyword 174 us (self 174 us)
import time:       - _heapq 189 us (self 189 us)
import time:     - heapq 400 us (self 211 us)
import time:     - itertools 137 us (self 137 us)
import time:     - reprlib 231 us (self 231 us)
import time:     - _collections 142 us (self 142 us)
import time:   - collections 3006 us (self 1456 us)
import time:     - collections.abc 191 us (self 191 us)
import time:   - types 518 us (self 327 us)
import time:   - weakref 577 us (self 577 us)
import time: - functools 4903 us (self 729 us)

patched:

import time:   - _functools 74 us (self 74 us)
import time:       - _operator 100 us (self 100 us)
import time:     - operator 460 us (self 361 us)
import time:     - keyword 174 us (self 174 us)
import time:     - itertools 139 us (self 139 us)
import time:     - reprlib 226 us (self 226 us)
import time:     - _collections 85 us (self 85 us)
import time:   - collections 2593 us (self 1512 us)
import time: - functools 3369 us (self 703 us)