Update map_type_to_rust_accessor to provide default values for each
type, which are used to generate helper methods that return a default
value when a field is missing.
Generate `which_<oneof>` methods and corresponding enums to handle
oneof fields in generated messages. Also add `has_<field>` helper
methods for all fields.
Update the generator to detect map fields and use MapFieldIterator.
Implement MapFieldIterator in the runtime to handle key-value pair
extraction and add write_map_entry to ProtoBuilder. Add tests to
verify that map-bearing messages generate and compile correctly.
charles