A couple commits to make life easier and more performant :)

Hi, still haven't got around reworking the NSCoding/NSCopying code, but here are the couple other niceties I've worked on.

One allows assigning numeric values to numeric properties (both NSNumber and primitives) that exist as strings on the dictionary. Also changed NSDateFormatter allocation to happen statically, but optimised to only instantiate those that are actually requested (big performance boost). Finally changed passing property_attributes_t by reference, which is a small performance boost but nice to have nonetheless since it was trivial.

The other one is a trio of handful macros that help writing key paths and mappings/custom transform, all validated and autocompleted by Xcode. :smile_cat: