Pry::WrappedModule: add module? and class? predicates

* module? whether the object is *strictly* a module (using instance_of?, does not match classes)
* class? must be a direct instance of Class (using instance_of?)
Delegate more methods from Candidate back to WrappedModule
Make show-source/show-doc fall-back to superclass

When performing show-source/show-doc on a class whose code
cannot be extracted we now fall-back to its first accessible superclass
and show its code together with a warning.

Example:

class Model < ActiveRecord::Base
end

show-source Model #=> shows ActiveRecord::Base code
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