org.json4s.ast.safe use String or BigDecimal, or should it be a sealed abstract class?

[mdedetrich]

There is currently debate at scala/slip#28 for what the internal representation for a org.json4s.ast.safe.JNumber should be. Currently its a BigDecimal, however there are valid issues with using BigDecimal

On approach from @djspiewak is to use a String, along with a private constructor that checks if the string is a valid number with a cached regex (we need to do this, as safe needs to guarantee that a JNumber is actually a valid number)

Another approach, used by https://github.com/argonaut-io/argonaut, is to have JNumber as a sealed abstract class with different number types as options. I am less open to this as it creates a lot of complexity, prefer the string approach with having a regex to check that its valid