Make hiffy_decode return a non-nested error

[mkeeter]

(staged on #660)

hiffy_decode returns a Result<std::result::Result<T, String>>, which is an awkward type.

We can instead use the HiffyCallError (renamed HiffyError), which distinguishes between internal errors (returned by the HIF program) and external errors (which occur when calling hiffy).

This simplifies most cases (where we want to return both inner and outer error), and adds a tiny bit of boilerplate to cases where we care about the difference – but that boilerplate is more clear than the nested type.