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When ranking consider whether a required context has *only* free variables instead of whether it has *any* free variables

The reasoning behind this is that unresolved instances with only free
variables are more likely in good results. For example, the user may
already realize that the function may be overloaded, and search for:

	avg :: [a] -> a

The intended function is overloaded, but the lack of general instances
should not devalue its result.

Make class arguments types instead of strings to be able to store classes with applied higher-kinded class variables

Add parameters to Cloogle.DB.Factory for whether parse warnings/errors of Clean.Doc should be traced to stderr

This is found for example in module frontend of the compiler:

	# dcl_mods & [module_n].dcl_common = common \\ common <-: ti_common_defs & module_n<-[0..]

It is for example used as an argument to `prepare_unification`. There
are cases however where this function is only called when unification is
requested by the user, and otherwise no `prepare_unification` call is
made.