This PR fixes `split` in the presence of metavariables in the target.
The fix consists of replacing an internal use of `apply` for
instantiating match splitters by a new, simpler variant `applyN`. This
new `applyN` is not prone to #8436, which is the ultimate cause for
`split` failing on targets containing metavariables.
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Co-authored-by: Sebastian Graf <sg@lean-fro.org>
Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>
