Include stats for each planner phase in EXPLAIN metrics

[hazefully]

This PR adds new stats to the PLANNER_METRICS struct produced for EXPLAIN SQL queries which reflect the number of tasks in each planner phase and the total time the tasks for each phase took to execute. It also adds a new lightweight Cascades debugger implementation which only keeps tracks of debugger events.

To achieve this, the PR includes the following changes:

• 2582ff8: This commit splits the cascades.debug.Debugger interface into two
  separate interfaces (StatsDebugger and SymbolDebugger) which both extend the base Debugger interface, in
  order to ensure that operations related to recording Debugger events (with the purpose of calculating stats
  based on the profiling of these events) make use of the StatsDebugger interface, and similarly operations related to
  registering different objects used within the planner make use of the SymbolDebugger interface. The State class which kept track of symbols and debugger events is also split into two different classes: EventState and SymbolTables.

• bdf2e16: This commit extracts the functionality of rendering the different stats for debugger events from the State class to a separate StatsViewer class.

• cf9848b and a9edd0f: These two commits include the main work of grouping debugger events by the planner phase they were emitted in, and exposing that grouping in the EXPLAIN planner metrics. As a result of this grouping the "Event Profiling" rendered by the StatsViewer class is now split by phase as well:

  

  In addition, the result set of an EXPLAIN query contains the expected planner metrics:

    { TASK_COUNT -> 248
    , TASK_TOTAL_TIME_NS -> 98967370
    , TRANSFORM_COUNT -> 61
    , TRANSFORM_TIME_NS -> 77887334
    , TRANSFORM_YIELD_COUNT -> 29
    , INSERT_TIME_NS -> 2012374
    , INSERT_NEW_COUNT -> 22
    , INSERT_REUSED_COUNT -> 6
    , REWRITING_PHASE_TASK_COUNT -> 44
    , PLANNING_PHASE_TASK_COUNT -> 204
    , REWRITING_PHASE_TASKS_TOTAL_TIME_NS -> 7869126
    , PLANNING_PHASE_TASKS_TOTAL_TIME_NS -> 91098244
    } 
  

• 2ee4763: This commit adds the new lightweight debugger implementation that only implements the StatsDebugger interface.

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Note: This PR previously took a different approach to expose the duration each planner phase took (12b5817), but after offline discussions we decided to make use of the existing Cascades debugger to expose these metrics.