dbg: introduce lj-ir, lj-jslots, lj-trace dumpers

This patch adds dumpers for a single IR instruction (`lj-ir`), as well
as for all bytecodes inside one trace (`lj-trace`). Its dump is quite
similar to the -jdump flag but also reports types of register operands
(`ref`, `lit`, `cst`) and operation mode (`N`, `A`, `W`, etc.).
The `lj-trace` command accepts optional /rs flags to dump registers
associated with IR and snapshots for the trace correspondingly.
The `lj-ir` command can be used for dumping IR constants as well.
The `lj-jslots` command dumps the content of `J->slot`. It is useful to
simplify debugging of `rec_check_slots()` assertion failures.

For LLDB value, the `__getitem__` metamethod now accepts bool keys.
Also, `__index__` is set to allow lldb.value to be used as an index
without explicit conversion to int. Old GDB versions (below 7.12) are
not supported because of the gdb.Value lacks the `__index__` metamethod
and can't be monkey-patched. The support for these versions may be added
by demand.

Part of tarantool/tarantool#4808

Author: Buristan