gdb: add rlist pretty-printer and 'tt-list' command

1. Add pretty-printer for 'rlist' type
Script holds table of possible rlists in tarantool. It recognizes
automatically actual type of entries and either expression refers to a
single items or to entire list. Then either single entry is displayed
(along with its index in the list) or the entire list (entry-by-entry).

2. Add command 'tt-list' that use the mentioned pretty-printer and
   provide additional functionality, namely:
- walking over the list (both directions)
- filter entries with predicate
- allow to explicitly specify list head and actual entry type if
  automatic recognition doesn't work
See 'help tt-list' for details.

Close #7731

NO_DOC=gdb extension
NO_CHANGELOG=gdb extension
NO_TEST=gdb extension