Add range notation

[nrc]

Add first class ranges: start..end, start.., and (implicitly) a whole range represented by a struct FullRange. The first two desugar to Range and RangeFrom structs, which implement Iterator and so allow, for example, for i in 2i..10 { }.

[nrc]

Part of #19148, see RFC.

[nrc]

Slight change of plan, the range structs have an iter method to get an iterator, rather than implementing Iterator themselves. Hopefully, one day, they'll implement some Iterable trait.

[nrc]

This means for i in (0..10).iter() { } is ugly as hell, but should look better with Iterable (although I've lost track if that is actually happening). The advantage is we don't need an extra cursor field in the Range structs, which seems like an important win.

[ftxqxd]

Why would we need an iter method? The current Range iterator doesn’t need one, and AFAICT it has the same representation as the one in the RFC, apart from an extra one field which wouldn’t be necessary for sugared ranges, which don’t have support for a step other than 1. The range structs could simply yield their lower bound and then increment it when next is called, right?

[nrc]

Yeah, that makes sense. It means we can't iterate over FullRange, but that is OK (I think), since it will have no surface syntax anyway.

[nrc]

I'm implementing syntax for Range and RangeFrom, depending on how RFCs rust-lang/rfcs#520 and rust-lang/rfcs#498 work out, we might need to add support for RangeTo later.