Rename 'StaticLabel' to 'StringLabel'.

This is an internal type so does not affect the API. The motivation
behind this name change is to make it clear that using a string as
a label is but one possible notion of static label, of which there can
be many.
Backward compatibility with GHC < 7.8.

Using the old Typeable API, rather than the new one that ships with GHC 7.8.