Add reset of Seedbank variables and FakeModel constant.

Signed-off-by: James McCarthy <james@thisishatch.co.uk>
Rename @runner to @_seedbank_runner.

Signed-off-by: James McCarthy <james@thisishatch.co.uk>
Add Seedbank::Runner#let and #let! methods for persisting across seeds.

Signed-off-by: James McCarthy <james@thisishatch.co.uk>