Decide on semantics for keys, foreign keys, and indexes

[ajcvickers]

Currently the model has the following first-class elements:

• Keys, one of which can be marked as the primary key, and which are assumed to have unique values
• Foreign keys, each of which references a key, and which can be marked as having unique values
• Indexes, which can be marked as unique

We should decide if:

• The use and placing of uniqueness flags is correct
• Whether an index object is always required if an index is to be created, or whether an annotation on a key or foreign key can indicate that it also represents an index
• Whether there is really a need to have separate elements for indexes and keys, or whether they can be combined.

Keep in mind that we explicitly wanted to have addressable items for use by Migrations so we don't have the same index naming/existence problems we had previously.

[ajcvickers]

Forgot I filed this. :-) Filed a new issue #700 based on outcome of design meeting discussions, so closing this one.