Fix crash when using append_inplace without batteries:export().
If you don't export batteries into the global namespace to stomp table,
then append_inplace would access a nil function because it's using table
instead of tablex.
We still use table inside init, but that's after we export to global
namespace.
Fix shadowing by using variables with a different name or _.
I don't think any of these warnings were actual bugs and fixed them to
maintain the same behaviour.
Run luacheck on CI to catch subtle errors.
Add a luacheckrc that ensures current code issues no warnings. We should
eventually try to remove many of these ignores.
