Simplify MSM calls for which only a subset of inputs are constant.

[TomAFrench]

We currently simplify any MSMs for which all the inputs are known however we can also do partial simplifications of MSM opcodes.

As ECadd/ECmul are commutative and associative, we can group together any constant terms and simplify them down to a simple curve point. This then just needs to be added to the result of the non-constant terms.