initial support for abstract casts using @:from and @:to functions

git-svn-id: http://haxe.googlecode.com/svn/trunk@5902 f16182fa-f095-11de-8f43-4547254af6c6
allow abstract to abstract cast

git-svn-id: http://haxe.googlecode.com/svn/trunk@5903 f16182fa-f095-11de-8f43-4547254af6c6