Make call builder with_arg work with IDLValue

[ericswanson-dfinity]

I recently had to add SyncCallBuilder.with_value_arg(IDLValue), because with_arg panics:

• IDLValue::ty() calls unreachable() : https://github.com/dfinity/candid/blob/master/rust/candid/src/parser/value.rs#L326
• called from here: https://github.com/dfinity/candid/blob/master/rust/candid/src/ser.rs#L28

It would be nicer if with_arg worked with IDLValue rather than panicking.

[chenyan-dfinity]

Yes, this is intentional. Receiving IDLValue means it's untyped, thus needing a different API. I'm planning to change IDLValue::ty() to return Type::Unknown, but it won't help your use case. One has to look at the value of IDLValue to know the type.

[chenyan-dfinity]

You can now decode function reference to candid::Func, so IDLValue is no longer needed.