Add a way to annotate union types? (semantically, things like `tag:array-key=>int`, `tag:array-key=>string`)

[TysonAndre]

It would be nice to have a way to add short-lived tag information within a function, optionally addable via phpdoc (param/return)

• This may be too slow to add to regular types.

add tag:array-key=>int, tag:array-key=>string, but with a more efficient representation (bit flag?) If exactly one of these is seen, then assume the array key is an int, and make that affect foreach, key, array_keys

• non-arrays shouldn't affect that?

• Warn if the tags exist and don't match expected tags.

• Erase it if the union type is merged with something lacking the tag

Other possibilities would be tag:array-shape => [0 => 'T1', 1 => 'int'] (can't be bit flag)

[TysonAndre]

Array keys and array shapes are part of the type system now, not as a separate annotation