Unification of templates, functions, decorators & projections

[markweitzel]

Templates are isomorphic with a function which takes types as arguments and returns types. The ability to use dynamic code to implement a template has been asked for in the past.

Likewise decorators are just functions which take a type and make changes to the type. This unification would enable implementing decorators purely in cadl without requiring TS.

Likely we just need to convince ourselves that we can do this in the future (i.e. have a design doc we can approve).

[nguerrera]

See also #521