Description:
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Since we now allow defining return value types for functions, one can also create definition for magic functions that make no sense, such as __toString() : float or __isset() : Closure. I think we should restrict such definitions - either to types that makes sense or in case of methods that aren't supposed to return anything useful, completely. We already do this for ctors, dtors and __clone but not consistently for other magics.

Specifically, the following should be true:

__clone - no return type
__isset - only boolean allowed as return type
__sleep - only array allowed as return type
__set - no return type
__toString - only string allowed as return type
__wakeup - no return type

Also, may make sense to restrict argument types (of course, not typing is always ok):

__call/__callStatic - only string for first and array for second arguments
__get/__set/__isset/__unset - only string for the first argument

__set_state - only array is allowed as argument type