Clean up parsing in static_ct_api

[lukevalenta]

• Use '&[u8]' (which implements Read) instead of Cursor where applicable
• Clean up SCT extensions parsing
• Clean up chain_fingerprints parsing
• Add enum for EntryType (X509Cert or Precert) and use where applicable
• Create PrecertData wrapper around fields that are only present in precerts, put an optional in StaticCTPendingLogEntry. The presence of the optional indicates whether the entry is a cert or precert.
• Make validate_chain return a StaticCTPendingLogEntry and remove ValidatedChain struct. And there's no need for validate_chain to return the raw issuers since the caller already has the raw chain.
• Use tlog_tiles::LookupKey instead of static_ct_api::LookupKey