Implement method `CppStringT::upper()`

[schmouk]

"str.upper()

Return a copy of the string with all the cased characters [4](https://docs.python.org/3/library/stdtypes.html#id15) converted to uppercase. Note that s.upper().isupper() might be False if s contains uncased characters or if the Unicode category of the resulting character(s) is not “Lu” (Letter, uppercase), but e.g. “Lt” (Letter, titlecase).

The uppercasing algorithm used is described in section 3.13 of the Unicode Standard."

[schmouk]

Completed.