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: Ie4a4a583fb120ff484a5ccf267ecd2a9c5a3902b
Reviewed-on: https://go-review.googlesource.com/2254


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