Add an exact Decimal number type

Add a first-class exact Decimal numeric type for business, financial, statistical, and database-oriented code. Decimal values use base-10 arithmetic so values such as 0.1 can be represented without binary floating-point artefacts. Objo source can write exact Decimal literals with a d suffix, for example 12.34d, while unsuffixed fractional literals remain Double values.

The type should provide arithmetic, comparison, parsing, formatting, explicit conversions, configurable rounding through the existing RoundingMode API, and exact database column and prepared-statement support.