Refactor and fix sect packing's bank iteration

Moving both pieces of related code together, as it were.
This also ends up changing the behaviour of scrambling,
where sections could “leak” out of the scrambling's bank pool
and resume being placed normally.

We have discussed it offline, and decided that this was a bug.
The test suite has been updated accordingly, which even gives
the occasion to move this test from a special case to the main
generic loop!

test/link/scramble-overflow.out

@@ -0,0 +1,2 @@
+FATAL: Unable to place "floating0" (ROMX section) anywhere within the 3 scrambled banks
+Linking aborted with 1 error