Add a CircleCI configuration that runs the tests and builds the Haskell API docs

it uses bash, at least.

```
--  While building package Cabal-2.4.1.0 using:
      /root/.stack/setup-exe-cache/x86_64-linux-nix/Cabal-simple_mPHDZzAJ_2.4.0.1_ghc-8.6.5 --builddir=.stack-work/dist/x86_64-linux-nix/Cabal-2.4.0.1 build --ghc-options "-hpcdir .stack-work/dist/x86_64-linux-nix/Cabal-2.4.0.1/hpc -ddump-hi -ddump-to-file -fdiagnostics-color=always"
    Process exited with code: ExitFailure (-9) (THIS MAY INDICATE OUT OF MEMORY)
    Logs have been written to: /root/project/.stack-work/logs/Cabal-2.4.1.0.log
```

Create a place for a real issuer

HTTP API for Voucher redemption