Reevaluate trace

[LinqLover]

Build an API to reevaluate a trace from an earlier point in time in a forked memory (#27).

In the context of the UI, this would be relevant for the following operations:

• accept contents to inspector field (currently changes the present state, unclear semantics)
• debug it from code pane/inspector field (could spawn another trace debugger)
• return value from context (not yet implemented)
• skip context (not yet implemented)

Technically, TDBRetracingSimulator could detect state changes and offer the client to reevaluate the trace automatically. See "retrace side effects" in the design process document.

Could either be solved via rematerialization (#23) into a copy or by melting a tracing simulator and a retracing simulator properly.