This patch adds a data formatter for NSDecimalNumber. The latter is a Foundation object used for representing and performing arithmetic on base-10 numbers that bridges to Decimal.
Side note: It would be slightly faster/elegant to use a StringRef instead of the char* for the comparisons.
Since the whole structure is one packed bitfield, an alternative implementation would be to read all 16b at once and memcpy to a local variable bitfield and let the compiler do the unpacking; that might be shorter. This version seems fine, too.