cast converted into its own ClauseElement so that it can have an explicit compilation

cast converted into its own ClauseElement so that it can have an explicit compilation
function in ANSICompiler
MySQLCompiler then skips most CAST calls since it only seems to support the standard syntax for Date
types; other types now a TODO for MySQL
then, polymorphic_union() function now CASTs null()s to the type corresponding to the columns in the UNION,
since postgres doesnt like mixing NULL with integer types
(long road for that .....)