ORM query helpers

[skearnes]

• Move base classes to a new base.py module
• Add query{_test}.py for defining ORM queries with fewer explicit joins

[codecov[bot]]

Codecov Report

Merging #655 (9924693) into main (8127d82) will increase coverage by 0.47%. The diff coverage is 97.75%.

@@            Coverage Diff             @@
##             main     #655      +/-   ##
==========================================
+ Coverage   74.51%   74.99%   +0.47%     
==========================================
  Files          23       24       +1     
  Lines        2723     2767      +44     
  Branches      540      570      +30     
==========================================
+ Hits         2029     2075      +46     
+ Misses        584      583       -1     
+ Partials      110      109       -1     

Impacted Files	Coverage Δ
ord_schema/orm/base.py	95.00% <95.00%> (ø)
ord_schema/logging.py	100.00% <100.00%> (ø)
ord_schema/orm/mappers.py	99.17% <100.00%> (+<0.01%)	:arrow_up:
ord_schema/orm/query.py	100.00% <100.00%> (ø)
ord_schema/orm/structure.py	91.25% <100.00%> (ø)
ord_schema/message_helpers.py	86.37% <0.00%> (+0.51%)	:arrow_up:

[skearnes]

I'm reworking the mappers to use programmatic class definitions and single-table inheritance; closing this for now since it will mostly be obsolete.