Provide alternative to proptest! macro to enable use of rustfmt

[lpil]

rustfmt is unable to format inside macros such as proptest!, so proptest test code has to be formatted manually, introducing irregularity and further burden on the programmer.

It would be great to have an alternative way to use proptest that didn't use this macro, possibly similar to quickcheck's annotation: https://github.com/BurntSushi/quickcheck#the-quickcheck-attribute-requires-rust-130-or-later

Thanks,
Louis