Force any_vec capacity to match length

[celinval]

Description of changes:

This is to avoid missing out-of-bounds access errors when dereferencing the vector with an index that is greater or equal to length.

Resolved issues:

Resolves #2759

Related RFC:

Call-outs:

I added some extra tests for exact_vec too since it is a public API.

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.

[zhassan-aws]

Thanks!