Consolidation of records and environments data structure

[jeromesimeon]

The main data structure for both records and environments are bindings (in Utils/Bindings.v). A couple of things could be improved:

• The internal representation (association lists, sorted without duplicates) bleeds through the rest of the code, and could be hidden better.
• There are two different kinds of lookups (lookup and lookupr) used in various parts of the code. It's unclear whether this is a necessary distinction (records vs environments?) or something that could be eliminated.