Implement DECIMAL data type

Implement the DECIMAL data type, and test for all built-in operators.

Do it using shifted integer values (or long, or short, depending on the required precision). For example, to represent 1234.56 as a DECIMAL(10, 2) we would use a 123456L (a long value because signed 10^10 requires 34.2 bits) and divide by 100 (2^10) on the way out.

In SqlOperatorBaseTest, convert public static final boolean DECIMAL = false to true, and fix the 9 tests where that constant is used.