Rethink mappings

[stephencelis]

I see a few issues with the way mappings are currently implemented:

1. They are automatically, categorically added to all NSManagedObject classes.
2. They are coupled to the NSManagedObject+ActiveRecord category.
3. They are unidirectional (there's no way to cast local properties back into remote ones).
4. They are declarative (and with @kattrali's Support generic value transformation in mappings #66, the dictionary can become quickly bloated).
5. They do a lot by default (auto-camelizing, for example).
6. They are a memory leak (we cache every unique remote key that we check for).

Let's explore ways to address these issues.

Proposal 1: Protocol-based Mappings

One thought is that we could provide a protocol, like NSCoding, that could be used to serialize/deserialize these mappings. E.g.,

@protocol ORCoding <NSObject>
+ (NSDictionary *)dictionaryFromRemoteDictionary:(NSDictionary)dict;
- (NSDictionary *)remoteDictionaryRepresentation;
@end

1. One would have to manually opt-in to the protocol.
2. Looser coupling, using conformsToProtocol:.
3. It would enforce bidirectional mappings.
4. Method logic is more imperative.
5. It's not included by default, so there are no defaults.
6. No memory leaks (that are our fault, anyway)!

[stephencelis]

Relates to #68.

[stephencelis]

/cc @supermarin

[dzenbot]

+1
That approach looks really nice

[supermarin]

@stephencelis I think that makes sense