Implement collections reform

[alexcrichton]

Tracking issue for rust-lang/rfcs#235, there are a number of sub-issues associated with this:

Backwards incompatible changes to make:

• Remove collections traits - @alexcrichton collections: Remove all collections traits #18474
• Add a Borrow trait - @aturon libs: Add borrow module, deprecate _equiv and friends #18910
• Add methods using the Borrow trait, deprecating _equiv methods - @aturon libs: Add borrow module, deprecate _equiv and friends #18910
• Ensure construction methods follow conventions -
• Ensure FromIterator is implemented - @gamazeps
• Ensure insertion methods follow conventions -
• Rename Extendable to Extend - @gamazeps Ensure Extend is implemented and renamed Extendable to Extend #18475
• Ensure Extend is implemented - @gamazeps Ensure Extend is implemented and renamed Extendable to Extend #18475
• Ensure deletion methods follow conventions -
• Ensure inspection/mutation methods follow conventions -
• Implement Index and IndexMut where appropriate - @alexcrichton - collections: Enable IndexMut for some collections #18445
• Ensure iteration methods follow conventions -
• Ensure capacity-related methods follow conventions -
• Implement bounded iterators for TreeMap
• Ensure set operations follow conventions -
• Ensure map operations follow conventions -
• Implement Deref for strings and vectors - @alexcrichton - collections: impl Deref for Vec/String #18443
• Ensure string api related methods are follow utf8/byte conventions -
• Add repeat to std::iter and the prelude - @jakub- Add a repeat function to the prelude #18468

Backwards compatible changes that will require additional language features:

• Add ByNeed and Predicate traits (needs negative bounds to work with unboxed closures)

Backwards compatible changes to make:

• Add Cow pointers - @aturon libs: Add borrow module, deprecate _equiv and friends #18910
• Add IntoIterator - @aturon
• Move for-loops to use IntoIterator - @jakub-
• Implement partitioning via FromIterator - @aturon

[alexcrichton]

My list may not be 100% accurate, so feel free to edit @aturon!

[Gankra]

@aturon how do you want to coordinate this work?

[gamazeps]

I ca give some help for the (not to hard) bugs :)
@gankro still working on your TODO, but it's a bit more complex than expected at first reading :p

[Gankra]

I assume it should read "implement deref on strings and vecs"?

[alexcrichton]

Ooops, indeed!