Improved debugger stack trace

[johanfylling]

debug.Session.StackTrace() currently returns a list of frames representing the complete set of query trace events from start of eval up to the current, paused debug event. This is both cumbersome to sift through and unexpected for the end user. A trace event doesn't correlate to what a user might expect would represent a stack "frame" in Rego. Closer to user expectations would be if each frame in the returned stack instead corresponded to the sequence of sub-queries (each with it's own, unique query-id) leading up to the current frame. Preceding sub-queries that has been exited (sequentially executed) should not be included in the stack trace.