Introduce template engine

[kaaveland]

It would be good to use a templating engine for rendering markdown now. We have the snapshot tests rig to help us make sure we don't change output unexpectedly, and it would reduce the complexity of the crate at this point. Both tera and handlebars look like good options that would slot nicely into eugene, since they can consume objects that are Serialize, which we already produce for json output.

[kaaveland]

mdbook, which we use for docs uses handlebars. I don't mind it and I'm pretty sure it delivers what we need:

• It accepts Serialize
• It can iterate over arrays with each
• If we need it, we have if
• We have partials
• We can include
• It has an option to enable no-escaping so that it won't eat SQL syntax or similar

[kaaveland]

70 solves this on the surface level, but there's still work to do, so keeping this open:

• Tests that the templates work with all combinations of output settings, especially for traces
• Refactoring the templates to use partials, particularly for the table
• Augment FullSqlStatementLockTrace with timing info, eg. how long did it hold locks for -- we're losing this info in #70 because I can't be bothered to fix it right now, but I'd like it back.

[kaaveland]

Closing this for now:

• ed98474b7304d9caf03283df2424ddac90663b58 adds back timing for locks
• 98e62013f56d61d4029dcb2a4ee9f406a01e5384 puts the lock table in a partial
• 4a2ff394840d483b78e09cfdfad5e5a629eeff86 adds the missing tests for summary and extra locks for the templates