Implement method `CppStringT::isspace()`

[schmouk]

"str.isspace()

Return True if there are only whitespace characters in the string and there is at least one character, False otherwise.

A character is whitespace if in the Unicode character database (see [unicodedata](https://docs.python.org/3/library/unicodedata.html#module-unicodedata)), either its general category is Zs (“Separator, space”), or its bidirectional class is one of WS, B, or S."

[schmouk]

Completed.
Fixed ìs_punctuation()`.