Simplify mpz.__format__(), part 3

[skirpichev]

Closes #358

[skirpichev]

from #188 (comment)

Benchmark	gmpy2	ref-v0.5.4	patch	int
format(123, '20d')	4.33 us	2.69 us: 1.61x faster	1.03 us: 4.20x faster	922 ns: 4.70x faster
format(-1234, '030x')	3.24 us	2.01 us: 1.61x faster	1.08 us: 3.00x faster	840 ns: 3.86x faster
Geometric mean	(ref)	1.61x faster	3.55x faster	4.26x faster

Details

# bench.py
import pyperf
from gmp import mpz
from gmpy2 import mpz as mpz2
#mpz = int
#mpz = mpz2

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)