Namegen.next_ident_away_in_goal: avoid names of all visible refs

[SkySkimmer]

Fix #3523

Considering the large stdlib changes this is probably not feasible, but let's see CI

[JasonGross]

Can we write a script to automate this process? E.g., have a setting that makes every top-level destruct and induction statement without an as clause whose naming behavior changes as a result of this PR emit a warning of the form "Name change upcoming, replace with destruct XXX as YYY to keep the old names", and then I can code up a python script that does the replacements based on the warning logs

[SkySkimmer]

Breaks too much to merge