DecodeToFloat64 : decode a chinese number string into Float64.

DecodeToInt64 : decode a chinese number string into Int64.

EncodeFromFloat64 : convert float64 into Chinese number 由于float64固然存在的精度问题，本函数可能不会特别精准。所以小数部分最多精确到6位。.

EncodeFromInt64 : convert int64 into Chinese number.

MustDecodeToFloat64 : decode a chinese number string into Float64 without error.

MustDecodeToInt64 : decode a chinese number string into Int64 without error.

Substr returns runes between start and stop [start, stop) regardless of the chars are ascii or utf8.