specialized string-like types with associated metatable methods #70

In the vein of Pythons regex strings (r "this is a regex"), it would be super-useful to add suitable metatables (and hence appropriate methods) to certain types of strings, as well as being able to recognize them with argcheck to help reduce coding errors.

For example, a file path could be declared as Path "~/.zemacs/init.zl", and automatically expand tilde expressions, and have methods for splitting, inserting etc by component much like the existing package.find and friends.