Introduces CaseInsensitiveStringSet, a sorted set of String that uses case-insensitive comparisons for ordering and equivalence.

The API is that CaseInsensitiveStringSet conforms to Comparable, CustomDebugStringConvertible, CustomReflectable, CustomStringConvertible, Decodable, Encodable, Hashable, Sequence, and SetAlgebra. A subset of RangeReplaceableCollection provides common functionality that the previous protocols don't cover.

The functions in "SortedSetMerge.swift" evaluate operations combining two sorted sets. rawSortedMerge(of: and:) vends the merger of the sets, giving an also-sorted result. Those results are expressed as one or two values with their category. Categories are: values only from the first sequence, values only from the second sequence, and values that the two sequences share. A shared vended element gives out the element value from both sources. This function doesn't necessarily need to be used directly, but it implements the actual user-facing functions.

sortedMerge(between: and: retaining:) vends a sorted sequence that applies the given set operation to combine the two given source sequences. doesSortedMerger(of: and: haveExclusivesToFirst: hasExclusivesToSecond: haveSharedElements:) checks the count of each result category, returning whether it matches the given pattern. A category can be checked for not being there, being there, or ignored.

The SortedSet type, in "SortedSetToo.swift", uses the above functions to implement SetAlgebra with an internal skip-list structure. SortedSet is not templated on Element directly, but through the protocol Orderable (in "Orderable.swift"). Orderable types specify their Element type and three functions that define ordering. You only have to implement the function that's an analogue to the less-than operation. The equal-to and greater-than operator analogues have default implementations that use the less-than function.

CaseInsensitiveStringSet wraps a SortedSet implementing an Orderable instantiation that uses case-insensitive comparisons from the Foundation system library. (The nil locale is used in `String.compare_: options: range: locale:)

When being printed, the regular version will print out the canonized casing for each element. (The hash implementation uses this form.) The debug version will print out the string that is actually stored.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.