Implement method `CppStringT::zfill()`

[schmouk]

" str.zfill(width)

Return a copy of the string left filled with ASCII '0' digits to make a string of length width. A leading sign prefix ('+'/'-') is handled by inserting the padding after the sign character rather than before. The original string is returned if width is less than or equal to len(s).

For example:
>>>

"42".zfill(5)
'00042'

"-42".zfill(5)
'-0042'"

[schmouk]

Completed.