Reference System.Memory and use it in math/raw

[Rob-Hague]

Describe your changes

This change represents an initial/example contribution towards #550. It adds a reference to System.Memory and uses it to deduplicate a number of internal code paths.

The change is kept isolated to one directory (src/math/raw) for sympathy with review. If this change is accepted, there is more that can be done in other layers of the library.

This also adds a reference to System.ValueTuple on net461 and a private reference to PolySharp. These are not necessary but allow using Index/Range syntax on lower targets.

How has this been tested?

Existing unit tests

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] I have kept the patch limited to only change the parts related to the patch
• [ ] This change requires a documentation update

See also Contributing Guidelines.