Use lifetime GAT for Source::Slice

[kmicklas]

This is required to write a Source impl for any type whose slice is a custom type and not a bare reference. For example, see ropey::RopeSlice.

Relates to #222.

[jeertmans]

Nice, thanks! I have read about GAT, but not actually used them.

do you have an example code that would now work with the new change, and that would not work previously?

[kmicklas]

I think the change in tests/src/lib.rs is indicative of the types of changes that would be required in user code. In general I wouldn't expect too many incompatibilities in user code (i.e. using default Sources) since the current Slice definitions and the new ones both resolve to &str/&[u8] for the appropriate lifetime.

The bigger source of breakages would be that this invalidates any Source impl.

[jeertmans]

Could you update Rust's MSRV so that it passes the tests? You can use cargo-msrv for that purpose :-)