Rename some of the functions in NameSet, to make the uniform with VarSet etc
For ages NameSet has used different names, eg. addOneToNameSet rather than extendNameSet nameSetToList rather than nameSetElems etc. Other set-like modules use uniform naming conventions. This patch makes NameSet follow suit. No change in behaviour; this is just renaming. I'm doing this just before the fork so that merging is easier.