Change RKObjectMapping's propertyMappings accessor method to simply return the internal array, rather than making a full copy on every access.  Several methods of RKObjectMapping itself call it like a regular accessor (to get the count, for example) and it gets called a tremendous number of times during mapping operations.  This saves a lot of memory during those operations, plus alloc/dealloc/retain/release/autorelease overhead.
Revert two unintended changes in the last commit

This reverts commit 54cbc9e4130382537eea95f78c24f084031adb07.
Change RKObjectMapping propertyMappings property to be immutable, to avoid having accessor be expensive (pay more of a price during setup instead, instead of the hot mapping code).
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Merge pull request #2066 from carllindberg/2065-ObjectMappingPropAccessor

Change RKObjectMapping's propertyMappings accessor method to simply retu...