Monoid instance for Exp Ordering

[Tjalle-S]

Description
Adds a Semigroup and Monoid instance for Exp Ordering.

Motivation and context
This follows base, and allows sorting on multiple criteria (in combination with the sorting algorithms in containers-accelerate.

How has this been tested?
No additional tests, but the existing tests pass.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[tomsmeding]

Thank you!