Safer `NSString`

[madsmtm]

• Bind the returned reference from as_str to the current AutoreleasePool plus the NSString (so e.g. 'return: 'pool + 'self). Should fix memory leak in INSstring.as_str() SSheldon/rust-objc-foundation#15.
  See also Make autoreleasepool take the pool as a parameter SSheldon/rust-objc#103 (comment).
  Done in Foundation ownership fixes #40
• Test using the Big List of Naughty Strings.
  • Moved to Start using property testing #46