Disabled digit separator in the "scan" command when scanning integers
and floating-point numbers.  This is the one place where existing code
may rely on number parsing to stop at an underscore.

Disallow underscore between the leading 0 and the radix specifiers
'x', 'o', 'b', and 'd'.

Added tests for disallowed underscore use and scan with underscores
between digits in the source string.