Fix all `reference` type aliases to match the return type of operator* exactly.

[ryanhaining]

Recent libc++ checks that an iterators reference type matches its operator*'s return type. This isn't the case across itertools and must be addressed. Testing with https://github.com/llvm/llvm-project/commit/36c746ca2d5b325a7ac64135c1ff8774c06ab34c