Bump hpack and test versions

• Require hpack ^>= 0.35 and pin extra-deps for stack build and tests.

Relax hpack version upper bound

• Relax hpack bounds to hpack >= 0.34.7 for stackage build.

Bump hpack and test versions

• Require hpack >= 0.34.7.
• Test with GHC 8.8.4, GHC 8.10.7, GHC 9.0.2 and GHC 9.2.2.

Bump hpack and test versions

• Require hpack >= 0.34.6.
• Test with GHC 8.8.4, GHC 8.10.7 and GHC 9.0.1.

Rewrite the README

• Rewrite the README, making it shorter and splitting some details about more
  uses and building into separate docs.
• Require hpack >= 0.34.4:

  $ dhall-hpack-cabal --version
  -dhall-hpack-cabal-0.5.2
  -hpack-0.34.2
  +dhall-hpack-cabal-0.5.3
  +hpack-0.34.4

• Require base >= 4.13, implying GHC >= 8.8.4.
• Test with GHC 8.8.4 and 8.10.4.
• Add files for different stack GHC versions.

  $ stack build --stack-yaml=stack/stack-8.8.4.yaml
  $ stack build --stack-yaml=stack/stack-8.10.4.yaml
  $ stack build --stack-yaml=stack/stack-9.0.1.yaml
  

• Remove the travis script and update github scripts with:

  - - uses: actions/setup-haskell@v1
  + - uses: haskell/actions/setup@v1
  
  - - uses: actions/cache@v1
  + - uses: actions/cache@v2

• Remove stale nix-related files.

Consistent Golden Tests

Use explicit dependencies to achieve consistent golden tests in all but stack-8.6.3.yaml.

Minor, bump in hpack version

• Regenerate golden files for the bump in hpack's version:

---- This file has been generated from package.yaml by hpack version 0.31.0.
++-- This file has been generated from package.yaml by hpack version 0.31.1.

Sorted Fields Pretty Printing

• Sort fields when pretty printing JSON and YAML.
• Add real world golden tests, using stack and hpack packages.

Split Executables

• Rename hpack-dhall to dhall-hpack-cabal.
• Add dhall-hpack-dhall, dhall-hpack-json and dhall-hpack-yaml executables.