Prevent instantiation of txns with 0 inputs or outputs

[prestwich]

Syntacitcally invalid transactions can be constructed. E.g. those with 0 inputs. This should be disabled

Should we update the transaction trait to new -> Result<Self, Self::Error> ?

[tynes]

the size for values of type 'Self' cannot be known at compile time: doesn't have a size known at compile time

[prestwich]

ya, to return Self in a trait you have to add a where Self: Sized constraint