Reviewed parser structs

[daladim]

This depends on #36, that must be merged first.

This PR changes the way Parsers work and own their data.

• This adds compiler safety guarantees (safe wrappers, alignment of allocated data)
• This improves the API (preferring an std::iter::Iterator over a getter with explicit indices, that must be bound-checked)
• This improves performance by:
  • efficiently caching "constant" data (i.e. that does not depend on a given EVENT_RECORD), so that it can be reused over various EVENT_RECORDs.
  • avoiding cloning some structures
• This adds a "Property not found" error, and fixes #38 (I was too lazy to extract this commit outside of this MR, as it would quickly conflict with other branches)
• This is one step towards a better multi-threading capability of this crate, by removing some &muts and introducing interior mutability

It is probably easier to review this PR commit-by-commit.

Also refs #18 and #25

[daladim]

Note: this PR, along with another one I haven't pushed yet (and that will depend on this one) drastically improves performance. Handling a message, checking its event ID, and parsing a property will be ~4 times faster (and even up to ~12 times faster if the event contains variable-length strings).