Add helpers to parse parameters.

[abizjak]

Purpose

Noticed in support that we don't seem to have a way to parse parameters using schema in the same convenient way that we do for events and return values.

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.