Prevent name/index collisions

[AntyMew]

Raises an AssertionError on any of the following:

• Name collision between message fields
• Index collision between message fields
• Non-positive field index
• Name collision between enum fields
• Value collision between enum fields
• Nonzero default enum value (in proto3)
• Name collision between any names defined in the current scope

Other changes:

• Adds test cases for the above
• Adds scope argument to _parse_enum to expedite collision checking
  • Takes a dict in the format of {token.name: token, ...} representing all names defined in the current scope (i.e. enum fields, message fields, and messages)
• Changes Message.fields and Enum.fields to dicts to expedite collision checking
  • Key represents the index/value of each field
• Adds Enum.default
  • Since dicts are unordered, indexing the first field for the default value is unreliable

[AntyMew]

Oops, thanks for the catch :S

[tburmeister]

This looks good! Thanks for the new unit tests as well. Can you bump the version to 0.8.6 in setup.py?

[tburmeister]

I'm happy to merge this if you think it's ready.

[AntyMew]

Thank you! Yep, ready to merge

[tburmeister]

Merged the changes and uploaded version 0.8.6 to PyPI. Thanks for your contribution, @AntyMew !