Add helpers to retrieve transactions and check their signatures from the chain

[abizjak]

Purpose

Make it easier to retrieve transactions and check their signatures when retrieveing them from the chain.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.