Add Support for Foreign Key Constraints

[Martin-Alexander]

Closes: #174

This fixes the following methods:

• add/remove_foreign_key
• add/remove_reference
• create_table
• change_table

The basic idea is that when we want to create a foreign key between a from_table and to_table we ask:

• Is to_table already schema qualified? If so, leave it as it.
• Do to_table and from_table both belong to the same schema? If so, leave them as is.
• Otherwise, we qualify to_table with the schema that it should belong to.

The methods #add_foreign_key and #remove_foreign_key are used under the hood by #add_reference, #remove_reference, and #change_table so the fix works for all of them.

A similar fix is added for TableDefinition#foreign_key that is used in #create_table

Knowing what schema a foreign key constraint should be on requires knowing what the database's "default" schema was (i.e., the current schema before we manipulated it by calling #on_temporal_schema or #on_history_schema). So this modifies #on_schema to keep track of that.

@tagliala Let me know what you think of this approach and if it looks good I can address some of the outstanding issues and clean up the code a bit.

Todo:

• Handle the case where #remove_foreign_key is given nil for the to_table keyword argument
• Refactor/clean up the repetition in the code
• Write tests to ensure schema quoting is properly handled
• Fix style offenses