Split (apply|diff)-base from (apply|diff)-term.
Split (apply|diff)-base from (apply|diff)-term.

This commit starts preparing for reusing Parametric.Change.Term in
Popl14.Change.Term.

Old-commit-hash: ca69607a3dac1cfb4c72f97c05baf74a862ef2bf

inc-lc/ilc-agda