Rework Schema API

[copiousfreetime]

Solving issue #16 showed that the way that Amalgalite represents the full schema of the connected database(s) is not proper. It needs to be more fully fleshed out. There are actually multiple schemas, one for the 'main' database, one for the 'temp' database and one for each 'attached' database.

The ability to represent all of these schemas is the goal. At the moment, there is just one 'Schema' and it is for the main, and the 'temp' schema is encapsulate within it. This is not the proper way to do this.

Changing the Schema API will result in a major version update as the API will not be backward compatible.