fix #959

Variable declarations using a type class will be subject to type inference similar to the one
already present in type coercions and the return type inference.
mostly fixes #1339

The compiler hangs were caused by the interaction of tyError and the instantiation caches.
For procs, the cache wasn't able to find previously compiled proc featuring tyError in the signature.
For types, the unresolved type parameters leading to tyError were not replaced everywhere leading
to endless replaceTypeVarsT recursion for cyclic types.

The fix is still not perfect, because the handling of tyError in other places in the compiler doesn't seem
to be complete and the first test case now results in internal error (still, much better than a hang blocking
your IDE).