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

Previous implementation treated TRIM as an SQL built-in function,
but in reality it is a special expression in SQL. The main difference
is that "TRIM" is not a name but a reserved keyword, so it can't
be wrapped into double quotes. Moreover, if we treat it as a function,
we need some special hacks to handle a complicated logic in its
arguments. Now TRIM is a separated expression with a custom parsing
logic.

At the moment we use String type for strings in Plan
structure. Since we frequently serialize, deserialize
and clone Plan, we want to reduce the time spent in
these functions. This commit substitutes String type
with SmolStr type, which has O(1) clone and, therefore,
speeds up cloning.