clash-spec-limit needs to grow exponentially

[kleinreact]

It seems like the following recursive structure causes an exponential unfolding of the generated structure, such that clash won't terminate in a reasonable amount of time.

f :: forall n. SNat n -> Unsigned 4
f n@SNat = case compareSNat n (SNat @15) of
  SNatLE -> at @n @(80 - n - 1) SNat (repeat @80 0)
  SNatGT -> f (SNat @(n - 2)) + f (SNat @(n - 7))

topEntity ::
  HiddenClockResetEnable System =>
  Signal System (Unsigned 4)
topEntity = pure $ f d77

Note that the code above also suffers from #2831, when choosing some constant that is smaller than 80.