'np' and 'pp' (next/previous prefix) iterators

Related dev. issue(s): https://github.com/tarantool/tarantool/issues/9994

Product: Tarantool
Since: 3.2
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_index/pairs/
SME: @ alyapunov

Details

Now there are two more iterators available: 'np' (next prefix)
and 'pp' (previous prefix). They work only in memtx tree. Also,
if the last part of key is not a string, they degrade to 'gt'
and 'lt' iterators.

These iterators introduce special comparison of the last part of
key (if it is a string). In terms of lua, if s is the search part,
and t is the corresponding tuple part, 'np' iterator searches for
the first tuple with string.sub(t, 1, #s) > s, while 'pp' searches
for the last tuple with string.sub(t, 1, #s) < s.

Comparison of all other parts of the key remains normal.

As usual, these iterators are available both in select and pairs,
in index and space methods.

Similar to all other tree iterators, they change only initial
search of selection. Once the first tuple found, the rest are
selected sequentially in direct (for 'np') or reverse (for 'pp')
order of the index.

For example:

tarantool> s:select{}
---
- - ['a']
  - ['aa']
  - ['ab']
  - ['b']
  - ['ba']
  - ['bb']
  - ['c']
  - ['ca']
  - ['cb']
...

tarantool> s:select({'b'}, {iterator = 'np'})
---
- - ['c']
  - ['ca']
  - ['cb']
...

tarantool> s:select({'b'}, {iterator = 'pp'})
---
- - ['ab']
  - ['aa']
  - ['a']
...

Requested by @ alyapunov in https://github.com/tarantool/tarantool/commit/96df090fe71a7152ddee105eca454efc0c096c28.

Start with the root document at reference/reference_lua/box_index/pairs/ and compare its iterator coverage with the issue description. Document the np and pp behavior, including memtx-only support, non-string fallback, select and pairs availability, and the examples shown here. Check the related development issue for implementation details and consider the work done when the page accurately describes and demonstrates both iterators.