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]

The old RUBY_GC_HEAP_INIT_SLOTS isn't really usable anymore as
it initalize all the pools by the same factor, but it's unlikely
that pools will need similar sizes.

In production our 40B pool is 5 to 6 times bigger than our 80B pool.

The test added in 90a80eb0 fails if Ruby is verbose, it outputs the
following line to stderr:

    RUBY_GC_HEAP_INIT_SLOTS=100 (default value: 10000)

There is an integer underflow when the environment variable
RUBY_GC_HEAP_INIT_SLOTS is less than the number of slots currently
in the Ruby heap.

[Bug #19284]

This commit adds RVALUE_OVERHEAD for storing metadata at the end of the
slot. This commit moves the ractor_belonging_id in debug builds from the
flags to RVALUE_OVERHEAD which frees the 16 bits in the headers for
object shapes.

The test could be flaky when a heap has below GC_HEAP_INIT_SLOTS number
of free slots because it would trigger a major GC and allocate more
pages.

The test is failing only on trunk-repeat50@phosphorus-docker. This
commit adds some debugging output to debug the failure.