stbx-service-rest: Decode and save transactions into the tx store

[epost]

This will be based on the code Marco introduced in #230.

• Create new spago package stbx-tx-store and move stuff from stbx-service-rest in there.
• Decode POST requests to /tx from hex instead of JSON, like the real tx service does.
  • Make Stbx.decode work from PureScript.
    • Fix tests for Statebox.Core, Statebox.Core.Execution, and Statebox.Transaction.Codec.
  • Decode POSTed { tx :: HexStr }'s tx field JSON
  • Decode this JSON to a TxSum
  • Define and use sendTxTxSum instead of Express's dynamically typed sendJson
  • Also compute the hash using Stbx
• Eliminate ExampleData.purs and its dependencies.
• tests
  • Add actual transactions to curl tests in scripts/test.sh.
  • Make them work using tx hex instead of expanded JSON
  • Replace by node.js tests in PureScript / JavaScript.
  • Introduce a test loader/runner that installs Stbx from stbx-js in the global scope.
  • Travis CI check passes even if a test suite in scripts/test.sh fails.
• Implement basic version of StateboxException from Jelle's https://github.com/statebox/cloud code.
  • Define corresponding TxError ADT with corresponding String-valued error codes, etc
  • Hook into some points
  • Create a dedicated sendTxError function instead of Express's untyped sendJson.