Support integer formatting types for mpz

[skirpichev]

• port CPython helpers
• use only public api in fill_number(), etc
• cleanup drop toupper/forward options

Closes #7

[skirpichev]

Benchmark	ref	patch
format(123, '20d')	2.15 us	2.03 us: 1.06x faster
format(-1234, '030x')	2.03 us	1.91 us: 1.06x faster
Geometric mean	(ref)	1.06x faster

Details

# bench.py
import pyperf
from gmp import mpz

runner = pyperf.Runner()
for v, f in [("123", "20d"), ("-1234", "030x")]:
    n = mpz(eval(v))
    v = "format(" + v + ", " + repr(f) + ")"
    runner.bench_func(v, format, n, f)