Expose RR and RR Header Len

[matje]

Related to issue #316

[miekg]

I think it makes most sense to do this in the same way as we did Copy,

dns/msg.go

Line 1893 in 85b661b

// Copy returns a new RR which is a deep-copy of r.

This would be a minimal I think Len() would then automatically work for RR_Header as that is an RR as well.

Also: currently the tests don't compile

[andrewtj]

Although not relevant to implementing this as func Len(RR) int, for future reference types_generate.go builds ztypes.go via go generate.

[miekg]

@matje are you making this change? Or do you want to closer this PR and open a new one?

[matje]

@miekg It's still on my todo list

[matje]

Easier to close this PR and create a new one with the easier fix, see #333