Make names const

[Neo-Zhixing]

std::ffi::CStr::from_bytes_with_nul_unchecked was const stabilized in Rust 1.59, meaning that the name() functions in extensions can now be const as well.

Alternatively we can do away with name() functions and use associated consts instead. However this would be a breaking change.

const NAME: &'static CStr = xxxxxx;

[Ralith]

Is this redundant to #590?

[Neo-Zhixing]

Oops, didn't see that #590 also did this. Closing now.

[MarijnS95]

I'd be happy to land both changes as separate PRs, tbh.