Add driver support for a new BSON type for decimal numbers, which will be added to MongoDB in version 3.4.

This type will use the decimal128 format as defined by the IEEE 754 standard and support up to 34 decimal digits and a range from -9.999999999999999999999999999999999×10^6144 to +9.999999999999999999999999999999999×10^6144. It also supports positive and negative zero, positive and negative infinity, and two forms of NaN (regular and signaled).

Decimal values will be treated like any other numeric type, and compare and sort correctly with other types based on actual numeric value. Operations on decimals are implemented in accordance with the standard, so a value of 0.10 will retain its trailing zeros while comparing equal to 0.1, 0.10000 etc.