Add SpreadType

Closes #72.

Note that we do not support the unofficial (unspecified) feature of the
RedHat compiler to allow spread types in a tuple type as well.

(A lot of this was actually done a few days ago – my local copy of
source/ceylon/ast/core/SpreadType.ceylon was last modified on Nov 24 –
but I only found the time to finish it today.)
Add tests for SpreadType