Implement method `CppStringT::find()`

[schmouk]

"str.find(sub[, start[, end]])

Return the lowest index in the string where substring sub is found within the slice s[start:end]. Optional arguments start and end are interpreted as in slice notation. Return -1 if sub is not found.

Note

The [find()](https://docs.python.org/3/library/stdtypes.html#str.find) method should be used only if you need to know the position of sub. To check if sub is a substring or not, use the [method `contains()`]"

[schmouk]

Completed.