Fix the `[pub]` hack in `prop_compose!`

[boustrophedon]

As of 1.30 it seems that you can now match on visibility keywords. Does this enable us to get rid of the [pub] hack in the prop_compose! macro? I haven't played with advanced macros so I'm not sure if it actually solves the problem. Also, it would make using the crate itself require a rustc >= 1.30.

[AltSysrq]

Duplicate of #10

Thanks for pointing out that it's now stable though, that had fallen off my radar. I'll see about adding support for the new matcher next time there's a release with breaking changes.

[Centril]

I'd recommend lumping in this change with the stabilization of the ? macro matcher (rust-lang/rust#48075 (comment)) at least as well as a change to edition 2018. This should make the macros somewhat simpler.

We might also want to consider experimenting with attribute proc macros now that they are stable, for example:

#[proptest]
fn foo(x: u8, ...) { .. }

tho it is unclear what to do about the x in strategy syntax as it would likely result in a parse error before attr proc macro expansion gets to run.

[AltSysrq]

At long last, this issue is fixed in 0.9.0. Unfortunately I wasn't able to preserve the old way of doing things, so when you upgrade you will need to remove the brackets around the visibility modifier.