[mypyc] Add primitive for int to ascii conversion (#21036)
Added a primitive to convert `int`s 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:
```python
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 |
