improves situation with auto-init

Symbol.typeSignature and Symbol.typeSignatureIn no longer auto-init.
Neither does Symbol.toString (something that I introduced a few commits ago).
However Type.toString still does, and that should be enough to get
robust prettyprinting with minimal risk. Details are in comments.
undeprecates typeSignature and typeSignatureIn

As per Jason's feedback, these are now undeprecated, being aliases for
info and infoIn. I haven't touched the newly introduced setInfo family of
methods, because I think that on internal level we don't need setTypeSignature
anyway.