DRILL-332 Changes to support decimal data type

Use SwappedByteBuf to allocate workspace variables.
Enable tests for decimal

Casting to Decimal from SQL

Make Decimal9, Decimal18 return BigDecimal for getObject()

Support returning BigDecimal for decimal sparse types

Dense to BigDecimal support

Float, Double to Decimal9, 18, 28, 38 and vice versa

Support for decimal literals.

Use holders for Decimal9 and Decimal18 literals

Add more tests to decimal dense sorting
Minor bug in dense sorting