Nasal: require 0o as prefix for octal numbers.

Nasal: require 0o as prefix for octal numbers.

Using just 0 as prefix for octal number can lead to confusion
where numbers could be interpreted the wrong way (oct instead of
dec).
Lets follow the same convention as Python 3 and Ecma 262 level 6
and use '0o' as prefix which can not be confused inadvertently.