Clarify the SRT building process

Clarify the SRT building process
Before: building a closure that would build an SRT given the top-level
SRT. It was somewhat difficult to understand the control flow, and it
may have had held onto some data structures long after they should be dead.
Now, I just bundle the info we need about CAFs along with the procedure
and directly call a new top-level function to build the SRTs later.