---------------------------------------------
	    Calculate the free vars of a type 'right'
	---------------------------------------------

	type C u a = u

Question: is 'a' free in 'C u a'?

I think the answer should be 'no'; see typecheck/should_compile/tc157.hs
for an example of why it matters.  This commit makes it so, and adds
comments to explain a dark corner in the zonking code.