The SignatureAndHashAlgorithm from TLS 1.2[1] is being changed to
SignatureScheme in TLS 1.3[2]. (The actual values are compatible
however.)

Since we expect to support TLS 1.3 in the future, we're already using
the name and style of SignatureScheme in the recently augmented
ClientHelloInfo. As this is public API, it seems that SignatureScheme
should have its own type and exported values, which is implemented in
this change.

[1] https://tools.ietf.org/html/rfc5246#section-7.4.1.4.1
[2] https://tools.ietf.org/html/draft-ietf-tls-tls13-18#section-4.2.3

Change-Id: I0482755d02bb9a04eaf075c012696103eb806645
Reviewed-on: https://go-review.googlesource.com/32119
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>