add new_range_api for RFC 3550

This includes a `From<legacy::RangeInclusive> for RangeInclusive` impl for convenience, instead of the `TryFrom` impl from the RFC.
Having `From` is highly convenient and the debug assert should find almost all misuses.

This includes re-exports of all existing `Range` types under `core::range`, plus the range-related traits (`RangeBounds`, `Step`, `OneSidedRange`) and the `Bound` enum.

Currently the iterators are just wrappers around the old range types.

Author: pitaj

library/core/src/lib.rs

@@ -403,6 +403,8 @@ pub mod panicking;
 #[unstable(feature = "core_pattern_types", issue = "none")]
 pub mod pat;
 pub mod pin;
+#[unstable(feature = "new_range_api", issue = "125687")]
+pub mod range;
 pub mod result;
 pub mod sync;