List refactoring (v6)

[danog]

Fixes a bunch of bugs all over the codebase.

[danog]

Wew, this PR triggered a huge amount of previously uncovered and broken edge cases, I fixed a bunch of bugs also present in master.
I've been thinking of merging this into master for v6, and then backporting a TList-compatible version (that removes TList-specific logic but keeps compatibility by converting plugin-originated TLists into the equivalent TKeyedArrays before handling, possibly directly inside the constructor of Union).

[danog]

The HasArrayKey assertion turned out to be completely broken and actually useless, for example when asserting array_key_exists($key, $arr) it asserted the existance of $arr['$key'] (not $arr[$key]), only to bypass a bug in class constant key assertions (which I also fixed in this PR).
It might be worth completely removing it.

Edit: nevermind it's only really used properly for class strings (still a bit weird though), reverted that part.

[danog]

Closing until the master branch becomes the v6, refer to #8820