crypto/x509: export raw SubjectPublicKeyInfo.
crypto/x509: export raw SubjectPublicKeyInfo.

The SPKI will probably be used for identifying public keys in future
HSTS specs.

R=bradfitz
CC=golang-dev
http://codereview.appspot.com/4485044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang