ec2: rough support for AWS::EC2::NetworkAcl.
add+use EmbeddedProperty marker interface.
specialize ResourceBuilder#addProperty(String, Object).
addProperty("blah", stringLike.getMap()) => addProperty("blah", stringLike).
introduce Derefable interface for resources and params.
remove calls to Derefable#ref() where possible.
ec2: simplify a SecurityGroup setter.
iam: simplify a setter.
remove two more uses of Derefable#ref().