Rename "reference block" to "execution block"

[mmagician]

general note for another discussion/PR: I wonder if the name "reference block" could be improved, I personally don't find it very intuitive: it's not clear what it refers to.

I think "execution block" would do more justice to its role.

Originally posted by @mmagician in #3737 (comment)

[crazywriter1]

I'd like to be assigned to this if nobody's on it. Plan: rename "reference block" to "execution block" consistently in docs and APIs where applicable. @mmagician