Fix bounds

[emilypi]

Hi @jprupp

This PR fixes the build matrix for secp256k1-haskell, which currently does not build on GHC 7.10, 8.0, or 8.2 due to a lack of bounds, and also addresses #34. You can futz with them however you like, but (at least) the lack of bounds on base16-bytestring caused a breakage due to a major version bump that changed the API. Here, I've generated conservative bounds using cabal gen-bounds.

It also fails for any LTS before lts-9.0, since unliftio and unliftio-core were not included in any previous LTS's. Fortunately cabal-install supports GHC 7.10 and unliftio-core, so if you're okay with not using stack, you can get away with keeping the current GHC support. Otherwise, I've done the fix assuming you like stack, and that includes supporting lts-9.0, which means i've lowered your cabal version, and bumped your base bounds to be GHC >=8.0.2 to accommodate the strict cabal-version and base requirements of the old LTS's.

I also noticed you're using unliftio as a dependency, but only using the typeclass. I've relaxed the dependency to use unliftio-core, which should make your build a little faster, save on CI costs, and generally reduce the carbon footprint of the project 😄

[jprupp]

I noticed you added limits to the Cabal file, but that file is generated from the package.yaml file. Would you do the changes there and then use Stack to generate a new Cabal file from it, and then commit everything? I will then merge. My apologies for not responding earlier.

[emilypi]

@jprupp You have the bounds, and your package is now up to date. I will not be making any more changes, but you are free to make commits to the PR to suit your convenience.