crypto/x509: implement crypto.Signer

Signer is an interface to support opaque private keys.
These keys typically result from being kept in special hardware
(i.e. a TPM) although sometimes operating systems provide a
similar interface using process isolation for security rather
than hardware boundaries.

This changes provides updates implements crypto.Signer in
CreateCRL and CreateCertificate so that they can be used with
opaque keys.

This CL has been discussed at: http://golang.org/cl/145910043

Change-Id: Id7857fb9a3b4c957c7050b519552ef1c8e55461e
Reviewed-on: https://go-review.googlesource.com/3126


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>