revised version of #546.

• makes t argument required following discussion there. not sure yet that this is the best, but I was almost there already so I just submitted this PR.
• benchmark results in the discussion of that PR that showed that creating a separate callable that skips argument processing had only a small performance advantage, so I decided to leave dynamics as a method.
• leaves code and usage of _rhs and _out unchanged in iosys. The idea is to leave them as fast internal methods and to have dynamics and output serve as more user-friendly versions where speed is of less importance.