Support JSON parameters

[limemloh]

Purpose

Support smart contract parameters in JSON format using the smart contract module schema.
Closes #94

Changes

• Add WaitUntilFinalized method on ConcordiumClient for waiting for transactions to finalize.
• Add Parameter.UpdateJson and Parameter.InitJson for constructing parameters for update and init transactions using JSON and smart contract module schemas.
• Minor changes to FFIError in lib.rs removing some manual From implementations.

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.

[MilkywayPirate]

Looks great!

Minor comments - probably most because of my ignorance of C#, so take it for what it is :-).