Add special case shrink pass for sums of integers

[DRMacIver]

We don't currently reduce tests that look like the following well:

@given(st.integers(0, 1000), st.integers(0, 1000))
def test(m, n):
    assert m + n <= 1000

In that we might end up with basically any pair (m, n) with m + n == 1001, when ideally we'd always end up with m = 1, n = 1000.

It'd be nice to normalise such a simple test. This PR adds a special case shrinking pass for tests that have sum lower bounds like this.

Full disclosure: This is me cheating at a benchmark. https://github.com/jlink/shrinking-challenge/blob/main/challenges/bound5.md