Automatically Load the hiffy_call result

[mkeeter]

(staged on #663)

This PR changes the return type from hiffy_call from Result<Result<Value, String>> to a Result<T: Load, HiffyCallError>. There are two changes here:

• We automatically unpack into a T: Load (instead of returning a Value)
• We collapse the nested outer / inner error types into an enum HiffyCallError, which represents both cases

This ends up being a bunch of churn, but is a significant cleanup overall.