cef_time_t callbacks not defined

cef3 callbacks returning `cef_time_t` are not implemented.  Trying to define them results in "unacceptable return type in foreign declaration" errors.  I believe this is because the Haskell FFI cannot return structure by value.  Since the CEF3 framework invokes these callbacks, they _must_ return a structure by value because CEF expects them to.  I do not know how to reconcile this, if possible.
