This PR introduces a `VestedTransfer` Trait, which handles making a
transfer while also applying a vesting schedule to that balance.

This can be used in pallets like the Treasury pallet, where now we can
easily introduce a `vested_spend` extrinsic as an alternative to giving
all funds up front.

We implement `()` for the `VestedTransfer` trait, which just returns an
error, and allows anyone to opt out from needing to use or implement
this trait.

This PR also updates the logic of `do_vested_transfer` to remove the
"pre-check" which was needed before we had a default transactional layer
in FRAME.

Finally, I also fixed up some bad formatting in the test.rs file.

---------

Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Bastian Köcher <git@kchr.de>