Fix an error in class to interface conversion
Use immutable constructors for standard Throwables

This allows Exceptions and Errors to be thrown even in immutable
contexts.
Add EOFException and use it in stdin::readLine()
Use IndexError in substring(); add substring(uint)
Fix string::indexOf()
string length -> property; add IllegalActionError
Add string::startsWith()
Add unicode and string::trim()
Add the Iterator interface
Add LinkedList to the standard library