tablex: Add deep & shallow for copy, overlay; remove redundant recursion; add tests

[idbrii]

Remove redundant recursion

Remove recursion from second loop of deep_equals. The first loop already checked for equality so the second only needs to check if the keys are there. Add test to validate.

Add deep & shallow for copy, overlay

BREAKING Implement the comment over copy():

• Replace copy with shallow_copy, deep_copy.
  • These copy functions copy any input type instead of requiring a table. That makes it easier to write generic code where you just want to copy some input without knowing anything about it. Like an event system, save system, etc.
• Replace overlay with shallow_overlay, deep_overlay.

Add corresponding tests.

Tests can be run with testy which is a low setup test framework -- it runs local functions prefixed with test_ and uses assert to detect failures.

Add github action to run tests on PR. See test output: success on idbrii@d48d4b0 fail on idbrii@d917e96. You can see that using vanilla assert() gives different output from batteries assert because testy provides a low-noise and non stopping

[1bardesign]

I'm actually a bit hesitant to have tests in the repo for size reasons; a lot of folks including myself will use the repo as a submodule and end up packaging the tests up along with the code if they do it "the straightforward way". The alternative is having them in an external repo like the examples though, and that seems likely to get noses out of joint as well.

In the interest of getting this merged, I'll raise an issue to talk about a long term solution to this.