extend event system to enable full inspection of match events

[tendrils]

this requires definition of a scan event type, holding a reference to the (text) block or fragment passed to the scanner. these need to exist until all handlers generated by the scan have finished executing.

[tendrils]

draft design showing the lifespan of references to data on the input buffer:

draft plan for making queue and event types more generalized in support of this and other work:

[tendrils]

initial implementation committed @3031176746d5049d5f2c5c55d70317f2bdbc02ae