[ opt ] Use IntMap in LHS.Unify.
[ refactor ] various things in LHS.Unify.
[ pointless opt ] Use IntMap in Epic.Injection.
[ Issue 1280 ] Insert hidden `unknown' arguments for parameters when quoting a constructor.
[ 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.
Merge branch 'maint-2.4.2'
[ tiny refactor ] and documentation of Telescope.splitTelescope.
[ Issue 1369 ] Fixed by correct raising of de Bruijn indices.
[ Issue 1369 ] Test case.
Merge branch 'maint-2.4.2'
[ fix ] bug in injectivity which could surface with --copatterns.
[ comment ] fixed typo in Polarity.hs
[ Issue 1366 ] fixed by clearing 'Forced' in LHS telescope.
[ Issue 1366 ] test cases.
Merge branch 'maint-2.4.2'
[ CHANGELOG ] Listed bug fixes for Agda-2.4.2.2.