Structurally prevent a number of common anti-patterns involving generic
signatures by separating the interface into GenericSignature and the
implementation into GenericSignatureBase. In particular, this allows
the comparison operators to be deleted which forces callers to
canonicalize the signature or ask to compare pointers explicitly.
The top-level PrintAsObjC now just covers import printing and the
near-literal prologue/epilogue text, while the decl printing (and
import collecting) is in its own file. No functionality change.
