Only use .container_descriptor if method exists
Don't try to .perl a non-PerlableAttribute
Move %WIRE_TYPE constant to PB::Binary::WireTypes
Bug fix encode-zigzag()
Add %!fields-by-tag and avoid non-field attributes

Two enhancements to PB::MessageClassHOW:

* Avoid non-field attributes when working with attribute lists by skipping
  all attributes containing a hyphen (field names can't contain them)
* Add a %!fields-by-tag hash for efficient field lookup when reading PBs
Track unknown fields in PB::Message base class
Start sketching full-message binary PB reading