Support annotations

[jonnor]

It should be possible to annotate a trace, with comments / metadata. This is especially useful when collaborating, as a developer can add their analysis/questions in a way that other tools can then display.
It may also be useful for semi/automated analysis tools to inject annotations, to point to specific areas of interest.

Annotations should be stored outside the events stream, as an event is considered immutable.
Possibly we should add an 'event id', to allow such references to be done in an unambigious way (even when trace is/has-been transformed). As a prototype one could use the array index though.