Add `implements` keyword for Classes

[markwpearce]

• Add a new keyword implements to explicitly say which interfaces a class conforms too.
• A class can explicitly implement multiple interfaces
• If a class uses the implements keyword, then validation is performed on the class to ensure that the set of its public members are compatible with all the interfaces referenced by the keyword.

Example:

interface Flier
    function takeOff(altitude as float) as boolean
end interface

interface Quacker
    sub quack()
end interface

class Duck implements Flier, Quacker    ' validation error - quack() is not included
    function takeOff(altitude as float) as boolean
       return false
   end function
end class