Generalize eq_rect proof in split1_firstn
Generalize eq_rect proof in split1_firstn

Caller needs to fill it in (it's a simple nia or Nat.mul_add_distr_r),
but the plus side is that it is now possible to rewrite <-.

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl