decimal: diallow infinity and NaN entirely.

While arithmetic operations do not return infinities or NaNs, it is
possbile to construct an invalid decimal value from strings 'Infinity',
'NaN' and similar. Some decimal mathematic functions may also result in
an infinity, say, ln(0) yields '-Infinity'.
So, add checks that the number is not a NaN or infinity after each
operation, so that the operation either returns an error, or a valid
finite decimal number.

Follow-up 6d62c6c1