Change some core mapping methods (in particular the per-attribute code) to use local variables instead of re-calling methods several times.  Any code called per-mapped-attribute is in the biggest hotspot of the mapping process and gets called a large number of times, to the point that avoiding unnecessary ObjC method calls begins to be noticeable.  So, for some of the core mapping methods (attribute ones in particular), use some local variables instead of re-calling methods on RKAttributeMapping, and similar things.
Merge pull request #2086 from carllindberg/2065-MapAttributeLocalVars

Use local variables to avoid repeated calls to methods