Add a merge algorithm for combining sorted sequences

[mdznr]

Description

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

Resolves #192

Detailed Design

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()
}

Documentation Plan

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

Test Plan

How is the new feature tested?

Source Impact

This change is purely additive.

Checklist

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