[SPARK-9759] [SQL] improve decimal.times() and cast(int, decimalType)

This patch optimize two things:

1. passing MathContext to JavaBigDecimal.multiply/divide/reminder to do right rounding, because java.math.BigDecimal.apply(MathContext) is expensive

2. Cast integer/short/byte to decimal directly (without double)

This two optimizations could speed up the end-to-end time of a aggregation (SUM(short * decimal(5, 2)) 75% (from 19s -> 10.8s)

Author: Davies Liu <davies@databricks.com>

Closes #8052 from davies/optimize_decimal and squashes the following commits:

225efad [Davies Liu] improve decimal.times() and cast(int, decimalType)