Decrease size of GlobalNamespace.Name and Ref objects

 * remove unnecessary Ref.name field
 * pack booleans & enum fields into a single bitset
 * replace GlobalNamespace.Name's ref map with either null/a single Ref/a lazily initialized map (we expect ~80% of Names have only 1 ref)

PiperOrigin-RevId: 551910017

Author: lauraharker