poly_constraints: update build

[mhuesch]

hi, thanks for the excellent project!

I just got poly_constraints to build with GHC 8.6.5. I cut all the version bounds out in order to do so, and am not sure what they should be. so, I don't really think this PR is mergeable, and maybe should serve more as a reference. hopefully it save some people some time 🙂

the changes were around Semigroup/Monoid, MonadFail, pretty operator precedence (code is untested; just typechecked), repline (API changes).

for reference, the cabal.project.freeze:

constraints: any.array ==0.5.3.0,
             any.base ==4.12.0.0,
             any.binary ==0.8.6.0,
             any.bytestring ==0.10.8.2,
             any.containers ==0.6.0.1,
             any.deepseq ==1.4.4.0,
             any.directory ==1.3.3.0,
             any.exceptions ==0.10.4,
             exceptions +transformers-0-4,
             any.filepath ==1.4.2.1,
             any.ghc-boot-th ==8.6.5,
             any.ghc-prim ==0.5.3,
             any.haskeline ==0.8.1.0,
             haskeline +examples +terminfo,
             any.integer-gmp ==1.0.2.0,
             any.mtl ==2.2.2,
             any.parsec ==3.1.13.0,
             any.pretty ==1.1.3.6,
             any.process ==1.6.5.0,
             any.repline ==0.4.0.0,
             any.rts ==1.0,
             any.stm ==2.5.0.0,
             any.template-haskell ==2.14.0.0,
             any.terminfo ==0.4.1.2,
             any.text ==1.2.3.1,
             any.time ==1.8.0.2,
             any.transformers ==0.5.6.2,
             any.unix ==2.7.2.2