String extension

This extension provides a wrapper around char* strings, providing a number of useful operators.
The purpose of this extension is to provide a simple demo of operator overloading.

Operators include

• String append: s1 + s2
• String repeat: s * i
• String equality: s1 == s2
• String index: s[i]
• String conversion operator: str(x)
• String representation operator: show(x)