Simplify Syntax

[b-studios]

Over the weekend, I took the time to cleanup the code base a bit. Quite a few changes accumulated:

• dropped extern effect since we have capabilities to take over this job (will be reintroduced later as extern interface, though).
• started to separate symbols from types. Previously, symbols like TypeVar have been a symbol and a type at the same time. By separating the two, it was possible to split symbols.scala into symbols.scala and types.scala making the distinction a bit more clear: (there is work left to do: CaptureUnificationVars are still symbols AND captures -- that's for another weekend).
• split record into Record + Construtor. This way data types do not give rise to record definitions for their variants, again making things a bit clearer (@phischu this should also help the LLVM backend).