Add the attribute macro to the prelude

[TomFryersMidsummer]

This is the most common item I use from proptest nowadays.

[rexmas]

Great to see that the proptest-macro is already being put to good use. However, even though this would be a convenience, I think it would send the wrong message about proptest macro's stabilization. It's not yet >= 1.0, while property test proper is.