The map requires plan is uniqueness among the syntax nodes, but
CTE violates this restrictions. So, migrate to the stack traversal.

Previously sbroad used ordinary Rust's `RefCell` for smart pointer behaviour.
It's pretty dangerous in tarantool environment, as it will be human responsibility to
check that no yield happens before `RefCell` is unlocked.

It causes problems with, for instance, cartridge's `apply_config` during live load - 
it needs to exclusively borrow the metadata, but it's already borrowed by ordinary insert requests.

- to_char(datetime, format), format is specified
as in `strftime` function:
https://man.freebsd.org/cgi/man.cgi?query=strftime&sektion=3