Only implement FusedIterator for TakeWhileInclusive when the inner iterator is fused

[patrickwehbe]

Fixes #1088.

TakeWhileInclusive sets its done flag only when the predicate rejects an element. It does not set it when the inner iterator returns None, so with a non-fused inner iterator the adaptor can return Some again after a None. The FusedIterator impl is currently unconditional (where I: Iterator), so that contract is violated.

This bounds the impl on I: FusedIterator, which is the same approach std::iter::TakeWhile takes (option 3 in the issue) and matches how the other conditionally-fused adaptors here are written (for example InterleaveShortest).

Note this is technically breaking: TakeWhileInclusive no longer implements FusedIterator when the inner iterator isn't fused. The previous impl was unsound in that case, so I left the changelog placement for release prep.

Added a fused_take_while_inclusive quickcheck test mirroring fused_interleave_shortest: not fused over the deliberately non-fused Iter, fused once the inner iterator is .fuse()d. cargo test, cargo fmt --check, and cargo clippy --all-features --all-targets all pass.