collections: Remove all collections traits

As part of the collections reform RFC, this commit removes all collections
traits in favor of inherent methods on collections themselves. All methods
should continue to be available on all collections.

This is a breaking change with all of the collections traits being removed and
no longer being in the prelude. In order to update old code you should move the
trait implementations to inherent implementations directly on the type itself.

Note that some traits had default methods which will also need to be implemented
to maintain backwards compatibility.

[breaking-change]
cc #18424
auto merge of #18474 : alexcrichton/rust/no-more-traits, r=aturon

As part of the collections reform RFC, this commit removes all collections
traits in favor of inherent methods on collections themselves. All methods
should continue to be available on all collections.

This is a breaking change with all of the collections traits being removed and
no longer being in the prelude. In order to update old code you should move the
trait implementations to inherent implementations directly on the type itself.

Note that some traits had default methods which will also need to be implemented
to maintain backwards compatibility.

[breaking-change]
cc #18424
refactor libcollections as part of collection reform

* Moves multi-collection files into their own directory, and splits them into seperate files
* Changes exports so that each collection has its own module
(that is, collections::treemap::{TreeMap, TreeSet} => collections::treemap::TreeMap, collections::treeset::TreeSet)
* Removes underscores from module/file names to provide more uniform naming
* Renames PriorityQueue to BinaryHeap
* Miscellanious fallout fixes

[breaking-change]