FloatingDecimal add final and use record

[wenshao]

In FloatingDecimal, the inner class PreparedASCIIToBinaryBuffer can be simplified with record, BinaryToASCIIBuffer can be added with final, and the fields isNegative/decExponent/digits of ASCIIToBinaryBuffer can be added with final.

Adding these is a better coding style, and it can also allow JIT to make more optimizations.

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