Stop needlessly wrapping object members and array elements in
variant variants.
Don't wrap object values in variants since the code is already using
vardicts for these. Return a variant array if a JS array contains values
of all the same type and don't wrap these in variants, else return
a tuple, which don't need to be wrapped either.
The only reason it was in the engine was so it could be used by both
the client and the web extension, without worrying about the
webkit2gtk and webkit2gtk_web_extension packages conflicting. However
it didn't really belong there, and added a dependency for the engine
on javascriptcoregtk which doesn't belong. So this fixes all that.
