Custom conversions between types

[reyleo]

Add feature to define custom conversion between types rather than fields

Proposal

@JMapTypeConversion(from=Long.class, to=BigDecimal.class)
BigDecimal fromLongToBigDecimal(Long longValue) {
...
}
@JMapTypeConversion(to=BigDecimal.class, from=Long.class)
Long fromBigDecimalToLong(BigDecimal decimal) {
...
}

Conversion of that type is only applied when no conversions by name are found