Add touch() and create_file() functions

[Beman Dawes]

A function with the functionality of the POSIX touch utility would be a great convenience. It meets both the need to update times and the need to create a file.

OTOH, it is a bit of a mishmash of logically separate features. Maybe what is really needed is a create_file() function that errors if the file already exists and a touch() function that errors if the file doesn't exist.

Alternatively, both could return a bool, true if successful, and not consider it an error if the file, respectfully, already/doesn't exist.