Fix massive slowdown in string formatting with str.format.

Example:
./python -m timeit -s "f='{}' + '-' * 1024 + '{}'; s='abcd' * 16384" "f.format(s, s)"

-> before: 547 usec per loop
-> after: 13 usec per loop
-> 3.2: 22.5 usec per loop
-> 2.7: 12.6 usec per loop