Support for attributes on declarations - Part II

[bettinaheim]

This PR contains basic resolution and handling in the symbol table.

The way it works:

• 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 (due to design reasons, meaning I want time to spec it)

All the intellisense etc should be enabled for attributes as well.

[cgranade]

Looks good to me, I think this is a great addition to the language and to the feature branch!