Using the length of a digest to infer the hash algorithm is somewhat
fragile - if we end up with multiple hash algorithms that share the same
digest length, things will break. Instead, pass more complete digest
information through to relevant functions and figure things out by
mapping the TPM hash algorithm to the appropriate Golang type.
This PR adds event log parsing logic. It's main goal is to require
validation at the same time as parsing, so structured events are always
verified against a quote. This new API replaces the exisitng "verifier"
package.
It's not a goal of this PR to parse the event data. This will be a
follow up, but since different users might want to parse different
events based on the OS, this API lets users of this package implement
custom event data parsing if they absolutely need to.
