This introduces a series of new classes for managing the state of

signatures. Signatures are what's stored every time a database evolution

is performed. They track the state of all the applications, models,

fields, etc. in the database, and are used to simulate operations and

compute differences. Previously, this was all managed by plain

dictionaries, which meant that the signature's format could never

structurally evolve (which will be necessary in the future), and these

new classes are designed to help address that.
The signature classes cover the main project signature, application

signatures, model signatures, field signatures, Django 1.11+ index

signatures. They feature the ability to create instances from their

respective instance types and from serialized data, and have the ability

to serialize back out.
Right now, these are actually very short-lived. They're built from other

state (stored state or in-process state) and are then serialized back to

a dictionary for all the evolution operations. Future changes will move

over to interfacing with the new signature classes directly, only

serializing when it's time to write out the state.

Unit tests pass for all databases and Django versions. Tested with

Python 2.7 and 3.6.