Remove MutableBox, make Box final

++immutability
Mark AsyncBuffer as final
Mark all Disposable implementations as final
Mark all signal/sequence methods as final

Subclasses should not be able to change the contract of these methods.
Fix naming of private properties in AsyncBuffer
Mark properties as final too
Make Refl into a struct

/cc @maxpow4h
Merge pull request #1385 from ReactiveCocoa/put-a-final-on-it

Put a @final on it