format for std::strings

create a function that will act as python's format() for str, filling given format string with arguments

this is a part of issue #15

The issue names no files, tests, or entry points. Read issue #15 first to understand the broader requirement, then locate the project’s std::string-related code before defining the function’s supported format syntax and argument behavior. Done means a C++ function fills a format string with the supplied arguments as requested.

Written by the indexing model from the issue text.