Remove Globals.priming.

Also remove references to it in code that isn't being tested in the
regression checking build-sequence, and so is probably bit-rotted.

Provide numvariant version of variant, which behaves like

with_flag(Globals.priming,SOME "") (variant avoids)

Document change/incompatibility in release notes.

Fix subscript nameStreams to start at 1 rather than 0

This was broken by f42dfacb21e and proofs do depend on
this (examples/ARM/arm6-verification/correctness/correctScript.sml in
particular).