Auto merge of #30884 - durka:inclusive-ranges, r=aturon

This PR implements [RFC 1192](https://github.com/rust-lang/rfcs/blob/master/text/1192-inclusive-ranges.md), which is triple-dot syntax for inclusive range expressions. The new stuff is behind two feature gates (one for the syntax and one for the std::ops types). This replaces the deprecated functionality in std::iter. Along the way I simplified the desugaring for all ranges.

This is my first contribution to rust which changes more than one character outside of a test or comment, so please review carefully! Some of the individual commit messages have more of my notes. Also thanks for putting up with my dumb questions in #rust-internals.

- For implementing `std::ops::RangeInclusive`, I took @Stebalien's suggestion from https://github.com/rust-lang/rfcs/pull/1192#issuecomment-137864421. It seemed to me to make the implementation easier and increase type safety. If that stands, the RFC should be amended to avoid confusion.
- I also kind of like @glaebhoerl's [idea](https://github.com/rust-lang/rfcs/pull/1254#issuecomment-147815299), which is unified inclusive/exclusive range syntax something like `x>..=y`. We can experiment with this while everything is behind a feature gate.
- There are a couple of FIXMEs left (see the last commit). I didn't know what to do about `RangeArgument` and I haven't added `Index` impls yet. Those should be discussed/finished before merging.

cc @Gankro since you [complained](https://www.reddit.com/r/rust/comments/3xkfro/what_happened_to_inclusive_ranges/cy5j0yq)
cc #27777 #30877 rust-lang/rust#1192 rust-lang/rfcs#1254
relevant to #28237 (tracking issue)

src/libstd/lib.rs

@@ -232,6 +232,7 @@
 #![feature(fnbox)]
 #![feature(heap_api)]
 #![feature(hashmap_hasher)]
+#![feature(inclusive_range)]
 #![feature(int_error_internals)]
 #![feature(into_cow)]
 #![feature(lang_items)]
@@ -246,7 +247,6 @@
 #![feature(optin_builtin_traits)]
 #![feature(placement_in_syntax)]
 #![feature(rand)]
-#![feature(range_inclusive)]
 #![feature(raw)]
 #![feature(repr_simd)]
 #![feature(reflect_marker)]