Speed up (and debug) serial json output

[kwalcock]

This is a draft because it depends on a processors update that needs to be published and there is still a small amount of test code remaining.

The most important enhancement is that the document.equivalenceHash is now cached and is not recalculated a bajillion times. Otherwise,

• the mentions serialization package file has been replaced by a regular file
• the mentions and related classes are not implicitly converted to MentionOps
• triggers are converted to Bio and Coref instances
• lots of duplicate code has been eliminated

This will be cleanup up and documented when processors is ready.

[kwalcock]

See #783, #784, #785.