Add explicit test case option

[echoumcp1]

Mirrors Hypothesis's example decorator. Provide an explicit test case as an input set:

hegel::hegel(
        [&] {
            auto x = hegel::draw(integers<int>());
            if (x == 10 || x == 20) {
                throw std::runtime_error("silly test failed");
            }
        },
        {.test_cases = 0, .examples = {{10}, {20}}});

x = 10 is tried first, then x = 20. The explicit test case is a list of draws. If value X is at the front of list when draw() is called, X is returned and popped from the front of the list (in actuality, the back, since the list is reversed). If the type of X does not match the type of the value that should be drawn, the test crashes.

If test_cases is 0, the server is never spawned and only the explicit test cases are used, otherwise the explicit test cases are tried first before asking the server for values.

Depends on #40