crypto/tls: expose names for CurveID and SignatureScheme

[izolight]

The constants for CurveID and SignatureScheme don't have any String() methods.
This makes printing these values (for example in log files) awkward as there is just a number.
To workaround this one can make a map to lookup from the constants to some friendly name (the constant name). As the constants already have custom types it would be best to implement the stringer interface for them in the standard library.
The idea is similiar to #30325
I created a first draft in #35297

[katiehockman]

/cc @FiloSottile

[gopherbot]

Change https://golang.org/cl/208226 mentions this issue: crypto/tls: add {SignatureScheme,CurveID,ClientAuthType}.String()

[rsc]

Adding to proposal minutes, seems headed for likely accept.

[rsc]

@katiehockman, @FiloSottile, are you in favor of this proposal?

[katiehockman]

@rsc I see no issues with doing this, and the rationale seems reasonable. The CL for this was pretty straight-forward too.

[FiloSottile]

Same, this looks good to me.

[rsc]

Based on the discussion above, this seems like a likely accept.

[rsc]

No change in consensus, so accepting.