Support closure proptests

[sunshowers]

The API could look something like:

#[propfuzz_closure]
fn foo() {
    // set up initial state...

    #[propfuzz(name = "test1")]
    |param: Vec<u8>| {
        // ... test
    };

    #[propfuzz(name = "test2")]
    |#[propfuzz(strategy = "state.strategy()")] param: StateParam<'_>| {
        // ... test
    };
}

Internally, this could be modeled through:

• the #[propfuzz_closure] attribute macro scanning through all the items and turning them into targets
• instead of a &mut TestRunner, pass in a &mut __PROPFUZZ_CLOSURE__foo where __PROPFUZZ_CLOSURE__foo is an autogenerated type:

struct __PROPFUZZ_CLOSURE__foo {
    test1: Option<TestRunner>,
    test2: Option<TestRunner>,
}

test1 and test2 provide access to individual test runners -- the Option is so that we can disable a test from running in the fuzzing context.

This is a fair bit of complexity and accordingly not very high priority unless we end up needing it.