Add Debug implementations where possible

[rust-itertools]

Add Debug implementations where possible

[bluss]

upstream libcore has Debug for iterators and adaptors now. the floodgates have opened!

[bluss]

Yes, we can do this better. Unfortunately multipeek can't derive either, it depends on std::iter::Fuse. We'll solve it.

[bluss]

Not sure about the best way to solve this. libstd iterators don't impl Debug either, so even if we do, it won't be available very often.

Might have an impact on compile time & library size (metadata / generics part) with very little gain.

[bluss]

It's a bit better in 0.5.5, lots of adaptors now have Debug. I ran out of steam to fix them all, so help wanted 😄

[Philippe-Cholet]

Based on cargo clippy -- -Wmissing_debug_implementations, the only missing debug implementation are in "src/groupbylazy.rs" for the ChunkBy, Groups, Group, IntoChunks, Chunks, Chunk structs. I doubt implement Debug for them would be useful and no one asked.
I relaxed some bounds in the past for some structs to be Debug.
I think we are good on the Debug front, I'm closing this.