Refactor `Atomic.value`
Refactor `Atomic.value`

This is the same way as `MutableProperty.value`.

calebd/ReactiveCocoa,ikesyo/ReactiveCocoa,victorlin/ReactiveCocoa,rpowelll/ReactiveCocoa,victorlin/ReactiveCocoa,smilypeda/ReactiveCocoa,smilypeda/ReactiveCocoa,andersio/ReactiveCocoa,sandyway/ReactiveCocoa,ohwutup/ReactiveCocoa,andersio/ReactiveCocoa,victorlin/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,rpowelll/ReactiveCocoa,sandyway/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,ohwutup/ReactiveCocoa,rpowelll/ReactiveCocoa,kevin-zqw/ReactiveCocoa,walkingsmarts/ReactiveCocoa,ikesyo/ReactiveCocoa,tzongw/ReactiveCocoa,tzongw/ReactiveCocoa,kevin-zqw/ReactiveCocoa,ohwutup/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,smilypeda/ReactiveCocoa,walkingsmarts/ReactiveCocoa,calebd/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,Juraldinio/ReactiveCocoa,Juraldinio/ReactiveCocoa,tonyarnold/ReactiveCocoa,walkingsmarts/ReactiveCocoa,tonyarnold/ReactiveCocoa,calebd/ReactiveCocoa,sandyway/ReactiveCocoa,Juraldinio/ReactiveCocoa,tzongw/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,kevin-zqw/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,tonyarnold/ReactiveCocoa,andersio/ReactiveCocoa