L1 RX: use queues instead of arrays and linear search for PUCCH, PUSCH, SRS, PRACH

This MR is an attempt to reduce the time L1 RX searches in array for the
next job to process, and instead use a queue. This queue is a FIFO,
because the various jobs (FAPI messages) to process come in order, and
need to be processed in order. The MR (hopefully, to be measured)
reduces the amount of time spent searching for the next UE (because the
next job is always at the beginning of the queue), and should scale
better for many UEs.

It does the following:

- introduce two helper libraries for (1) Frame.Slot calculation (sfn_t),
  already introduced in !3521 (merged) commit 3102068e, and (2) a ring
  buffer with fixed size
- use sfn_t and ring buffer (gNB->pucch_queue) to remove the linear
  array for PUCCH (gNB->pucch)
- use sfn_t and ring buffer (gNB->pusch_queue) for some PUSCH lookups.
  Because we need to still store PUSCH contexts, gNB->pusch is still
  there
- use sfn_t and ring buffer (gNB->srs_queue) to remove the linear array
  for SRS (gNB->srs)
- use sfn_t and ring buffers (gNB->prach_ru_queue and
  gNB->prach_l1rx_queue) to remove the linear array for PRACH
  (gNB->prach_list)
- some minor cleanups, e.g., additional loops over the PUSCH array,
  using const, using pointers instead of indices, etc