Mutable arguments and lazy definition of sampled_from interact poorly

[DRMacIver]

I discovered an unpleasant surprise when debugging #2505

Consider the following two tests:

from hypothesis import given, strategies as st

@given(st.data())
def test_mutability_1(data):
    x = [1]
    s = st.sampled_from(x)
    x.append(2)
    assert data.draw(s) != 2


@given(st.data())
def test_mutability_2(data):
    x = [1]
    s = st.sampled_from(x)
    assert data.draw(s) != 2
    x.append(2)
    assert data.draw(s) != 2

Currently the first one fails and the second one passes.

Why? Well because in the first one the lazy strategy containing the values of x is instantiated at the point of the draw after the append(2). in the second one it's instantiated before it.

I think sampled from is the only place where this is currently a significant issue, so this can probably be fixed by removing the defined_strategy from sampled_from, but we need to handle some of its special casing seperately to make sure it has a good repr then.