Refactor map<_,_> format decision in gpb_compile
Refactor map<_,_> format decision in gpb_compile

Refactor the decision to use maps or 2-tuples for map<_,_> type fields.

Instead of repurposing the get_records_or_maps_by_opts/1 function for
this decision, use a new get_2tuples_or_maps_for_maptype_fields_by_opts/1
to decide.

So far, the option semantics to gpb_compile:file are unchanged.

mmmries/gpb