Speed up setting the className method for accessor classes

For whatever reason class_addMethod is absurdly faster than
class_replaceMethod, and the name of an accessor'ss base class can never
change, so we don't need to be using replace. Makes creating new RLMRealm
instances roughly twice as faster.
Throw an exception when objects are used on the wrong thread

Only enabled in debug builds so it's currently totally pointless.
Speed up RLMArrayLinkView fast enumeration

Pull some of the lookups outside the main loop as was already done for
RLMArrayTableView. Makes fast enumeration of array properties about 50% faster
and now the same speed as enumerating results.
Remove unnecessary int overflow checks

NSUInteger and size_t should always be the same size, so converting between
them can't result in an overflow. Speeds up assigning short strings to
RLMObject properties by about 20% because the compiler couldn't optimize out
the message send for getting the string length.