Add RCL() macro for creating a signal with a constant value

For expressions like:

 * `RCL(5)` instead of `[RACSignal return:@5]`
 * `RCL(value)` instead of `[RACSignal return:@(value)]`
 * `RCL(CGSizeMake(2, 3))` instead of `[RACSignal return:MEDBox(CGSizeMake(2, 3))]`

Requested by @alanjrogers.