*: add simpleLevelIterator, reduce merging levels in external iter

Currently, we create a new merging iterator level for each file
passed into NewExternalIter. This is unnecessary for most use-cases
of creating ExternalIters around lots of sstables, as we can externally
guarantee that many of those sstables won't have overlapping points
with each other. We can have the caller pass this knowledge
by specifying a [][]sstable.ReadableFile where each sub-slice
obeys level invariants for files within it, and is also already
sorted by user keys.

This change makes the interface change to allow for the above
optimization, and also adds a `simpleLevelIter` that implements
forward iteration within a single "level". For files that don't
contain range deletes, we shove all the point iters into one
`simpleLevelIter`, greatly reducing merging iterator levels
and speeding up its operations by a lot.

Fixes cockroachdb/cockroach#83051.