Optimize `JsonParser.getDoubleValue()/getFloatValue()/getDecimalValue()` to avoid String allocation

[cowtowncoder]

With #1230 done, we can tackle one problem that may have been a regression: the fact that even when following JsonParser methods are used:

• getDecimalValue()
• getDoubleValue()
• getFloatValue()

there will still be a String allocation even if that is not really needed for deferred decoding (like JsonParser.getNumberValueDeferred())
This may be a regression to address other problems with too-early binding of type.