API to reset all; API to wrap all

The `init_types_collection()` function installs the tracing callbacks and sets the filename filter, but doesn't reset the globals containing collected data. Maybe it should also do that. Or maybe we need a separate API to do that. (Currently the pyannotate unit tests reset a whole bunch of globals at the start of each test -- this should become a single call.)

Second, `with collect_types.collect()` only calls `resume()` and `pause()` -- we could use another context manager that calls `init_types_collection()` and `dump_stats()` as well.

(Note that the non-context-manager APIs are also important, e.g. for use in test fixtures, where there are traditionally separate `setUp()` and `tearDown()` methods.)