[mypyc] Add primitive for int to ascii conversion

[p-sawicki]

Added a primitive to convert ints into bytes objects suitable for bytes format strings, eg. b'%d' % my_int. Works the same as the existing CPyTagged_Str primitive used for formatting str objects, except it returns a bytes object instead of str.

Currently only the bare %d specifier is supported, other integer specifiers and %d with options go through the regular CPython handling of format strings.

Results in around 23% reduced runtime in this microbenchmark:

import time

t0 = time.time()
for i in range(10_000_000):
    val = b'%d' % i
print(time.time() - t0)

Results:

setup	runtime (s)
interpreted	0.90
mypyc master	0.92
mypyc PR	0.69

[JukkaL]

Nice! I saw a 1.7x perf improvement when I did the benchmark compuation in a function (to avoid slow globals access).