External call hyper-parameter checks

[Aurel300]

(From #72)

External calls may optionally have hyper-parameters specified.

call! ext.someFunc() // no hyper-parameters
call(value: Wei(100))! ext.somePayableFunc() // value hyper-parameter
call(gas: 200)! ext.someFunc() // gas hyper-parameter
call(value: Wei(100), gas: 200)! ext.somePayableFunc() // both

(The hyper-parameters may be specified for all call modes, i.e. default call, forced call!, value-returning call?.)

• [ ] check that only value and gas are ever specified
• [ ] check that neither value nor gas is duplicated