Rollup merge of #42134 - scottmcm:rangeinclusive-struct, r=aturon

Rollup merge of #42134 - scottmcm:rangeinclusive-struct, r=aturon

Make RangeInclusive just a two-field struct

Not being an enum improves ergonomics and consistency, especially since NonEmpty variant wasn't prevented from being empty.  It can still be iterable without an extra "done" bit by making the range have !(start <= end), which is even possible without changing the Step trait.

Implements merged https://github.com/rust-lang/rfcs/pull/1980; tracking issue https://github.com/rust-lang/rust/issues/28237.

This is definitely a breaking change to anything consuming `RangeInclusive` directly (not as an Iterator) or constructing it without using the sugar.  Is there some change that would make sense before this so compilation failures could be compatibly fixed ahead of time?

r? @aturon (as FCP proposer on the RFC)