Issue Description:

// Array, ArraySlice, ContiguousArray:
public var capacity: Int

// Dictionary, Set:
public init(minimumCapacity: Int)

// Dictionary, Set, RangeReplaceableCollection:
public mutating func removeAll(keepingCapacity keepCapacity: Bool = false)

// RangeReplaceableCollection:
public mutating func reserveCapacity(_ minimumCapacity: Int)

Is it possible to support all of the above APIs in:

• Arrays.swift.gyb (Array, ArraySlice, ContiguousArray)

• HashedCollections.swift.gyb (Dictionary, Set)

• StringCharacterView.swift

• StringUnicodeScalarView.swift

For example, it might be useful to get/set the capacity of a dictionary.

Currently, only the `removeAll` method is common to all types.