fix(mutable.Filter): clear stale elements to prevent leak

[Emil-Ka]

Fixes #842

mutable.Filter shifts elements left but leaves the tail of the original slice untouched, causing ghost duplicates when iterating over the original variable.

Before:

doc := []el{{"foo", 234}, {"bar", 345}, {"baz", 111}}
mutable.Filter(doc, func(e el) bool { return e.Key != "bar" })
fmt.Println(doc) // [{foo 234} {baz 111} {baz 111}] ← duplicate baz

After:

fmt.Println(doc) // [{foo 234} {baz 111} {Key: Val:<nil>}] ← tail zeroed

Fix zeros out the tail after filtering (same approach as slices.Delete in stdlib). Applied to both Filter and FilterI.

[d-enk]

Because there is no problem with duplicates (just wrong usage)

In my opinion, title a unfortunate

Something like this would be better

fix(mutable.Filter): clear stale elements to prevent leak