Codegen all yield expressions, even if they don't have a matching block argument
Some refactors in specs for not requiring prelude, and some typos fixes
Fixed #67: instances that never get instantiated get their fields typed as int8. Calls that are never instantiated raise "untyped exception", but that should never happen in safe code. On the other hand, breaking from a block unconditionally doesn't work right now (but that's not very useful).