Function decimal

Smart constructor for valid arbitrary precision numeric values.

In our context, a valid arbitrary precision number is a finite numeric value represented as a :class value.

In general, the given value will be converted into a :class value if:

1. the value is valid number excluding NaN, Infinity and -Infinity, and
2. the value is a string value and its sanitized representation can be parsed into a valid number excluding NaN, Infinity and -Infinity.