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 <[email protected]>