Implement free variable–computing functions (for things besides DType) correctly

[RyanGlScott]

In #105 (comment), @goldfirere observed that many of the free variable–computing functions in L.H.T.D.FV (that I added in #95) were pretty slipshod. For instance, here is one case of fv_letdec:

th-desugar/Language/Haskell/TH/Desugar/FV.hs

Line 79 in 8495349

go (DSigD _ t) = fvDType t -- A function's type can't mention its name

This completely falls over on this expression:

let foo :: a -> a; foo x = x in foo y

The only free variable in this expression is y, but due to the case above, fvDExp would claim that a is a free variable. Ack!

This was far from the only issue with these functions, so I will ultimately be ripping them out in #105 once that is approved. But I do have a reason for wanting them: they will be necessary to fix goldfirere/singletons#339. This issue serves as a reminder to revisit the implementation of these functions so that they can be done correctly next time.