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.
