Fix CT serialisation

[AlCutter]

This PR fixes an error in the serialisation of Static CT API leaf data.

The ctonly package could do with some golden tests, I've added #420 to track that.

[phbnf]

I expanded all the structures from RFC6962 and https://c2sp.org/static-ct-api, and I think that this PR steers away from the specs. What do you think? (apologies for poor formatting)

LeafData

struct {
           uint64 timestamp;
           LogEntryType entry_type;
           select(entry_type) {
               case x509_entry: ASN.1Cert;
               case precert_entry: PreCert;
  struct {
          	      opaque issuer_key_hash[32];
                   TBSCertificate tbs_certificate; <-- according to rfc6962, this is precert without the precert bit etc, i.e defangedTBS, not the Raw precertificate. [static-ct sets the defangedTBS into leaf.Certificate](https://github.com/transparency-dev/static-ct/blob/d5b947a46312391263762e2855e0df7456fe17b5/handlers.go#L539), leaf.Precertificate is the raw Precertificate. ccl: this field should be leaf.Certificate
                } PreCert;
           } signed_entry;
           CtExtensions extensions;
       } TimestampedEntry;
	TimestampedEntry timestamped_entry;
	select (entry_type) {
		case x509_entry: Empty;
		case precert_entry: ASN.1Cert pre_certificate; --> According to c2sp/static-ct-api, this is the precertificate submitted, i.e [what static-ct sets in leaf.Precertificate](https://github.com/transparency-dev/static-ct/blob/main/handlers.go#L537), not the issuer precertificate
	};
	Fingerprint certificate_chain<0..2^16-1>; ---> If this is a Precert entry, [the first hash of that chain will be the hash of PrecertSigningCert](https://github.com/transparency-dev/static-ct/blob/main/handlers.go#L498-L500), which itself can be fetched from the issuer storage.
	} TileLeaf;

MerkleTreeLeaf

struct {
           Version version;
           MerkleLeafType leaf_type;
           select (leaf_type) {
               case timestamped_entry: TimestampedEntry;
		   struct {
                   uint64 timestamp;
                   LogEntryType entry_type;
                   select(entry_type) {
                       case x509_entry: ASN.1Cert;
                       case precert_entry: PreCert;
            struct {
          	                opaque issuer_key_hash[32];
                             TBSCertificate tbs_certificate; <-- According to rfc6962, this is precert without the precert bit etc, i.e defangedTBS, not the Raw precertificate. [static-ct sets the defangedTBS into leaf.Certificate](https://github.com/transparency-dev/static-ct/blob/d5b947a46312391263762e2855e0df7456fe17b5/handlers.go#L539), leaf.Precertificate is the raw Precertificate. ccl: this field should be leaf.Certificate
                         } PreCert;
                   } signed_entry;
                   CtExtensions extensions;
               } TimestampedEntry;
           }
       } MerkleTreeLeaf;