State union should merge Argument validators.

[proofit404]

class A1State(State):
    v1 = Argument(is_integer)

class A2State(State):
    v1 = Argument(greater_than_7)

NewState = A1State & A2State

The v1 argument in the NewState would be both an integer and > 7.

Checks:

• success
• validation error in one condition (error message shows in what state it was defined)
• in composition of thee layers

[proofit404]

Superseded by #722