Touch-up for PEP 451
PEP 454

* Add Trace, Traceback and Frame classes: generated on-demand read-only views
  of traces
* Add a new StatisticDiff class and add new Snapshot.compare_to() method.
  The method return a list of StatisticDiff instances and replaces the
  compare_to parameter of Snapshot.statistics()
* Remove Statistic.size_diff and Statistic.count_diff attributes
* Remove get_traces()
PEP 454: remove Snapshot.timestamp attribute
[PEP 451] Mark as Accepted!
PEP 454: remove filters on tracing

* Remove add_filter(), get_filters(), clear_filters()
* Remove the default filter on tracemalloc.py
* Give more explanation on the impacts of set_traceback_limit()
PEP 454: merge two "functions" sections