This commit adds key force_incremental_marking_finish_count to
GC.stat_heap. This statistic returns the number of times the size pool
has forced incremental marking to finish due to running out of slots.

There is a `time` key in GC.stat that gives us the total time spent in
GC. However, we don't know what proportion of the time is spent between
marking and sweeping. This makes it difficult to tune the GC as we're
not sure where to focus our efforts on.

This PR adds keys `marking_time` and `sweeping_time` to GC.stat for the
time spent marking and sweeping, in milliseconds.

[Feature #19437]

RDoc was making every usage of the word "GC" link to the page for GC
(which is the same page).