prio-queue: rename .nr to .nr_ and add accessor helpers

prio-queue: rename .nr to .nr_ and add accessor helpers

Rename the .nr member to .nr_ so that callers outside prio-queue.c
that directly reference .nr get a compilation error.  This catches
both existing misuse and future in-flight topics.

Add prio_queue_size() for callers that need to know the element count
and prio_queue_for_each() for callers that need to walk all elements.

Convert all external .nr users:
 - Loop conditions: use prio_queue_size(), prio_queue_get(), or
   prio_queue_peek() as the loop condition
 - Array iterations: use prio_queue_for_each()

Signed-off-by: Kristofer Karlsson <redacted>
Signed-off-by: Junio C Hamano <redacted>