Critical Description
Replace HiveDecimal implementation that currently represents the decimal internally as a BigDecimal with a faster version that does not allocate extra objects
Replace HiveDecimalWritable implementation with a faster version that has new mutable* calls (e.g. mutableAdd, mutableEnforcePrecisionScale, etc) and stores the result as a fast decimal instead of a slow byte array containing a serialized BigInteger.
Provide faster ways to serialize/deserialize decimals.