Memory and DOM element leaks in magic

[bep]

This is related to #2126 -- but different enough to warrant its own issue.

This issue covers two different leaks, but they're closely related.

Both are demonstrated here with reference to possible patches:

https://alpine-testlab.netlify.app/leak2-magic/

injectMagics holds a reference to el that never gets released

I have tested with this patch bep@6c14dfc -- which works for my simple test case, but I notice that injectMagics is also used elsewhere.

magics' effects, e.g. in $watch, is never released

Which becomes a problem when they hold a reference to a DOM element (which they often do).

I have tested this patch: bep@9dfe0c4

That works. However, it only works for $watch and it may make the making of custom `magics´ harder than it should.