Fix the inconsistency around naming

[fffej]

For some reason I called Table, TableDefinition and now everything has a Definition suffix. This makes things a bit sucky.

Concrete tasks

• Move the definitions into a definitions module
• Use explicit imports and exports everywhere to make the coupling clearer

[fffej]

Still need to do explicit imports / exports.

[fffej]

See #29 and some quick and dirty commits on master.