[ opt ] Use IntMap in LHS.Unify.
[ refactor ] various things in LHS.Unify.
[ pointless opt ] Use IntMap in Epic.Injection.
[ new ] Utils.Permutation.inversePermute
[ comment ] Commented Rules.LHS.instantiatePattern.
[ new ] Alternative implementation of Rules.LHS.instantiatePattern
Does not work yet (behaves different from original).
[ refactor ] Parametrize Syntax.Internal.Pattern over variable type.
[ refactor ] Alt. impl. of Internal.Pattern.patternsToElims

First, label pattern vars by their de Bruijn index.
Then, turn pattern into term by a simple non-monadic traversal.