ck_ring: Expose *_size variants to type-specialized ck_ring.

Added:
 - CK_RING_ENQUEUE_SPSC_SIZE
 - CK_RING_ENQUEUE_SPMC_SIZE
ck_ring: Correct CK_RING_SPMC_ENQUEUE_SIZE typo.
regressions/ck_ring: More improvements and _size coverage.

SPMC path exercises random delays. Coverage exists for
the new *_enqueue_*_size