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

Requested by @alanjrogers for expressions like:

 * `RCLBox(5)` instead of `[RACSignal return:@5]`
 * `RCLBox(value)` instead of `[RACSignal return:@(value)]`
 * `RCLBox(CGSizeMake(2, 3))` instead of `[RACSignal return:MEDBox(CGSizeMake(2, 3))]` – or, without [Archimedes](https://github.com/github/Archimedes), `[RACSignal return:[NSValue valueWithSize:CGSizeMake(2, 3)]]`

~~Based on the implementation of [MEDBox](https://github.com/github/Archimedes/blob/92e3bf36d1a5a28a3c23c428192b70742de01ed0/Archimedes/NSValue%2BMEDGeometryAdditions.h#L11-L24).~~