Redefine GridLength as readonly struct to prevent defensive copies

[h3xds1nz]

Description

Since GridLength is immutable by design, we should adjust it to readonly struct. This will also prevent defensive copies. It will also improve code quality. This is not a breaking change.

• I've also simplified the ctor a bit, which will also offer a very minor performance benefit (it does not matter).
• Minor cleanup of the usual IEquatable functions to save precious code lines and simplify it.
• The value was being set to 0.0 in case of Auto but when queried, it is set to 1.0.
  • I couldn't see a reason for it, the only thing that made sense was the hash code creation but the hash code is not unique even for basic values. It is too easy to get duplicates. This should be fixed later, I'll create a tracking issue.
  • See Redefine FigureLength as readonly struct to prevent defensive copies #9887 for a small conversation regarding this with miloush.

Customer Impact

Possibly a bit of perf and a clear directive that GridLength and all its methods are not adjusting the state of the struct.

Regression

No.

Testing

Local build, base testing, type creation, etc.

Risk

I do not asses any, since the struct is immutable and this is not a breaking change. It is a change to a public type but such changes have been done previously in runtime, see for example #97421.

Microsoft Reviewers: Open in CodeFlow