[mypyc] Implement boxed constant integers as literals

[JukkaL]

This avoids (potentially) creating a new int object
every time we evaluate an integer literal in a context that
requires a boxed value. In any case this reduces the number
of C calls required.

This speeds up this microbenchmark by about 60%:

def f() -> None:
    for j in range(1000 * 1000):
        a = []
        for i in range(10):
            a.append(10)

In more realistic workloads the impact is hard to
measure and is likely below the noise floor.