oveloaded find() for string and char pointer

Added support for searching string in bc::string find().

Tried adding doctest.

Please review.
So each doctest name needs to be unique (i.e. only one block of coded surrounded by `//! [string_find]`). For the `find()` method I'd have a block which created the input string, created the pattern string, and then called `input.find(pattern)`. In the method documentation in basic_string.hpp I would then describe what the pattern string was and what the `find()` method would return for the example.