rts: Allocate non-moving segments with megablocks

Non-moving segments are 8 blocks long and need to be aligned.
Previously we serviced allocations by grabbing 15 blocks, finding
an aligned 8 block group in it and returning the rest.
This proved to lead to high levels of fragmentation as a de-allocating a segment
caused an 8 block gap to form, and this could not be reused for allocation.

This patch introduces a segment allocator based around using entire
megablocks to service segment allocations in bulk.

When there are no free segments, we grab an entire megablock and fill it
with aligned segments. As the megablock is free, we can easily guarantee
alignment. Any unused segments are placed on a free list.

It only makes sense to free segments in bulk when all of the segments in
a megablock are freeable. After sweeping, we grab the free list, sort it,
and find all groups of segments where they cover the megablock and free
them.
This introduces a period of time when free segments are not available to
the mutator, but the risk that this would lead to excessive allocation
is low. Right after sweep, we should have an abundance of partially full
segments, and this pruning step is relatively quick.

In implementing this we drop the logic that kept NONMOVING_MAX_FREE
segments on the free list.

We also introduce an eventlog event to log the amount of pruned/retained
free segments.

See Note [Segment allocation strategy]

Resolves #24150

-------------------------
Metric Decrease:
    T13253
    T19695
-------------------------