Port implementer-defined records to Puma

[axch]

The Lite implementation is in backend/lite/records.py Puma would need

• a similar representation for record objects
• the ability to translate records to and from stack dicts
• matching definitions of constructors and accessors (this piece should be a performance optimization only, if the stack dict translation works)

Then turn on the unit test of refs in Puma

[axch]

Tests that are broken in Puma because of this:

• test_observe.py
• test_inc_eval.py
• test_make_sym_dir_mult.py
• test_references.py