Export TU constructor MkTU for external combinator construction.

I am using this library for manipulating the GHC AST, which has holes. I have created  a first-pass version of stop_tdTU and full_tdTU, but to do that I needed to use MkTU.

See https://github.com/alanz/HaRe/blob/e010f1a84df0675c894d6bcd95bc7f9cfe08173d/src/Language/Haskell/Refact/Utils/GhcUtils.hs#L297 

Is this the best way to dodge the holes?

What is the best way to manage the GHC-Specific combinators? I see three options

1. Leave it buried in the bowels of HaRe
2. Expose it in a hackage library on its own
3. Include it in Strafunski.