Support value/type namespaces on identifier links

Identifier links can be prefixed with a 'v' or 't' to indicate the value or
type namespace of the desired identifier. For example:

-- | Some link to a value: v'Data.Functor.Identity'
--
-- Some link to a type: t'Data.Functor.Identity'

The default is still the type (with a warning about the ambiguity)