CCIP-009: Rename map for readability

[whoabuddy]

The improved VRF implemented in CCIP-009 comes from one of @LNow's Clarity Projects.

In that file, which is what CCIP-009 was based on, the map used for saving the random integer at a given block height is BlockRnd.

While implementing this in the production code for CCIP-009, I'm inclined to rename this to something more readable like RandomUintAtBlock.

The other functions get-rnd and get-save-rnd will stay the same, and the remaining functions were converted from read-only to private.

If that is approved then CCIP-009 needs to have the map name updated throughout for consistency.