Use PEP 757 C-API to import/export integers

[skirpichev]

Closes #14

[skirpichev]

Benchmark	export-ref	export-patch
mpz(1<<7)	843 ns	400 ns: 2.11x faster
mpz(1<<38)	941 ns	395 ns: 2.38x faster
mpz(1<<300)	1.71 us	652 ns: 2.63x faster
mpz(1<<3000)	9.14 us	2.49 us: 3.66x faster
Geometric mean	(ref)	2.64x faster

Benchmark	import-ref	import-patch
int(mpz(1<<7))	681 ns	183 ns: 3.73x faster
int(mpz(1<<38))	762 ns	222 ns: 3.43x faster
int(mpz(1<<300))	1.54 us	554 ns: 2.78x faster
int(mpz(1<<3000))	8.30 us	2.29 us: 3.63x faster
Geometric mean	(ref)	3.37x faster

benchmarks code

# export-bench.py
import pyperf
from gmp import mpz


runner = pyperf.Runner()
ints = "1<<7", "1<<38", "1<<300", "1<<3000"

for v in ints:
    n = eval(v)
    v = "mpz(" + v + ")"
    runner.bench_func(v, mpz, n)

# import-bench.py
import pyperf
from gmp import mpz


runner = pyperf.Runner()
ints = "1<<7", "1<<38", "1<<300", "1<<3000"

for v in ints:
    n = mpz(eval(v))
    v = "int(mpz(" + v + "))"
    runner.bench_func(v, int, n)