Support generic declaration of sig (delegate)

[hopscc]

If you try and declare a sig as a generic the compiler currently gives a parse error
e.g.

sig Function<of TParam, TReturn>( param as TParam) as TReturn

gives

error: Encountered OPEN_GENERIC when expecting an identifier.

This should be allowed.

See longer sample in ticket:271 (Torial)
C# code at  http://labs.kaliko.com/2010/03/cache-functions-in-c-using-memoization.html