With locations

[bartveneman]

Todo

• store locations for values
• store locations for aggregate lists
• store locations for specificities

Refs #298

Notes:

• This setups seems to consume a lot of memory and that should be addressed, but I'll tackle that separately;
• The implementation is under an unfriendly __unstable__uniqueWithLocations property when opted-in via the new useUnstableLocations flag. That should be enough warning signs that this isn't ready for prime time yet;