[stdlib] Dictionary/Set enhancements

A variety of enhancements from SE-154 and SE-165:

- Custom Keys and Values collections for Dictionary
- Two sequence-based Dictionary initializers
- Merging methods for Dictionary
- Capacity checking and reserving for Dictionary and Set
- Type-specific filter(_:) methods for Dictionary and Set
- A mapValues(_:) method for Dictionary
- A grouping Dictionary initializer

Author: natecook1000