sp-core: Rename VrfOutput to VrfPreOutput (#2534)

This will make more sense after
https://github.com/paritytech/polkadot-sdk/pull/2524 since the
schnorrkel type for VRF outputs is also renamed in the latest version.
Can be reviewed independently though.

Can be merged after https://github.com/paritytech/polkadot-sdk/pull/1577
so that there is less pain for @davxy.

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/frame/babe/src/lib.rs

@@ -384,7 +384,11 @@ pub mod pallet {
 							});
 
 							public
-								.make_bytes(RANDOMNESS_VRF_CONTEXT, &transcript, &signature.output)
+								.make_bytes(
+									RANDOMNESS_VRF_CONTEXT,
+									&transcript,
+									&signature.pre_output,
+								)
 								.ok()
 						});