Add a `merge` algorithm for combining sorted sequences

Adds a simple merge algorithm for combining two sequences that are known to already be sorted by the same criteria

Include any additional information about the design here. At minimum, show any new API:

extension Collection {
    /// The new feature implemented by this pull request.
    func newFeature()
}

How has the new feature been documented? Have the relevant portions of the guides been updated in addition to symbol-level documentation?

How is the new feature tested?

This change is purely additive.

[x] I've added at least one test that validates that my change is working, if appropriate
[x] I've followed the code style of the rest of the project
[x] I've read the Contribution Guidelines
[ ] I've updated the documentation if necessary

apple / swift-algorithms