Fix opaque type warning.
Update behaviour definition.

This removes the behaviour_info function, which means that we can only
support R15A and later. If this is a contentious change, it is easy to
add a rebar.config.script to support both styles, although it is less
clean.
Fix bug in pncounter increment-by-negative.

Also, export and use the riak_dt:actor() type.
Simplify gcounter to remove dialyzer warnings.
Fix type error on pncounter to_binary/1.
Fix enable flag behaviour compliance.
Fix disable flag behaviour compliance.
Fix conflicting binary tag on LWWReg.
Fix OD-flag behaviour compliance.
Fix OE-flag behaviour compliance and all flags EQC integration.