When generating evolutions, mutations and database operations need to

track certain state, specifically indexes on tables. These were done

through a "database signature" object, which was a nested set of

dictionaries lacking any formal API. It had problems, namely that it

didn't prevent things like indexes without a name (which was being

generated in some cases), and was hard to maintain. It also wasn't named

appropriately, since it wasn't actually a signature of the database's

state like the other signatures.
This change introduces a new DatabaseState class, which is the

replacement for the old database signature dictionaries. This has a

formal, documented API for tracking tables and indexes, and handles

checking that the data going into it from callers is valid. It also

handles some of the types of lookups that we used to need various

utility functions for (some of those still remain as part of this

change, and will be removed in a future one).
The database_sig references had to change to database_state

throughout the codebase, which is a large part of why this change is

spread across so many files.

Unit tests pass for all Python versions on all database types for

all versions of Django.