ABT.subst avoids inspecting/rebuilding subtrees that can't contain the target variable
ABT.subst avoids inspecting/rebuilding subtrees that can't contain the target variable

This sped up the unison-shared tests from 8-9s to 5-6s on my machine

unisonweb/platform,nightscape/platform,unisonweb/platform,nightscape/platform,unisonweb/platform,nightscape/platform