Update regex-syntax to 0.8

[JustusFluegel]

Update regex-syntax to 0.8 and regex to the corresponding version. This would be nice as proptest is the only crate that still requires regex-syntax v0.7 in some of my projects and I would appreciate not having to compile it in that version anymore.

I would also be open to try to create a pr but I am not very familiar with the proptest codebase so I don't know how that would go. Feel free to tell me if that is welcome tho.

[matthew-russo]

Addressed with:
#386