Document and test Readonly "our" declarations.

[ronaldxs]

"our" declarations seem to work with Readonly much like "my", with the expected differences. It would be helpful for the documentation to formally mention "our" declarations as a feature but there don't seem to be matching tests. I have started work on documentation and testing here: https://github.com/ronaldxs/readonly/tree/doc-test-our. It is not ready for a PR but hopefully shows some progress. The tests in t/general/our.t do not use eval like other Readonly test files and tests for limiting updates of "Readonly our" declared variables would need to be added.

[vsespb]

+1