Replace struct impl of Invariant by Arbitrary

[celinval]

Description of changes:

We have recently realized that implementing Invariant of Option, Result and array has a limitation. This is not sufficient to generate arbitrary values of types that only implements Arbitrary.

Thus, implement Arbitrary instead.

Resolved issues:

N/A. Related to #1394

Call-outs:

Testing:

• How is this change tested? New tests

• Is this a refactor change?

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[celinval]

I forgot to mention that I did a few experiments with check_both test that I added. I changed the test to create an array with 100 elements instead.

For the arbitrary array, the performance impact with the changes in this PR was small. I tried a different implementation using MaybeUninit (similar to this example: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#initializing-an-array-element-by-element) since this would be the safest, and there was a significant penalty on the verification time. Here is the data I collected:

Verification Time:

Before Changes	After Changes	MaybeUninit
10.0s	10.7s	12.7s

[tedinski]

I guess I don't object after sleeping on it. I think we'll reverse course later, but it's probably not important.

Please hold off on merging until after today's release maybe?