Support for declaration attributes

[bettinaheim]

This is the final merge of the feature branch back to master, adding support for declaration attributes.

• any user defined type can be marked as attribute
• marking it as attribute does not prevent using it just like any other user defined type
• it is marked by attaching the "Attribute" attribute defined in the core namespace to it
• an attribute annotation then consists of a compile time constant of such an attribute
• such annotations for now may only be attached to declarations (types, functions, operations)
• for now they are disabled on specializations

[bettinaheim]

I'll merge this since the bulk of the work has been reviewed on the feature branch.