Skip reflection for most types

[miekg]

Make the reflection types a black list:

CAA, HIP, IPSECKEY, NSEC3, NSEC, SIG, TSIG, URI,

All other types don't use reflection anymore.

Return a pointer to the header when there is no rdata, this restores old
behavior. The rest of the conversion mostly hangs on getting size-hex
right, but then packStruct and packStructValue and the unpack variant
can be killed.

Generate pack and unpack for all embedded types as well.

[miekg]

@asergeyev this works, except for:

% go t                                                                                                                   ~/g/src/github.com/miekg/dns cleanup-noreflect +
--- FAIL: TestPrivateByteSlice (0.00s)
    privaterr_test.go:84: offset is not matching to length of Private RR: 17!=40
    privaterr_test.go:89: got error unpacking ISBN: dns: buffer size too small
FAIL
exit status 1

I'm afraid I've changed some (hidden) semantics on the Len() stuff. Could you check wth is going on here? Thanks!

[miekg]

@asergeyev nvm, the pack for the privaterr type didn't pack the header. Working. Now failing elsewhere, but not as severe.