Add new struct to the `expression` variant:
```c++
// A value serialized with the internal (latest) cql_serialization_format
struct constant {
cql3::raw_value value;
data_type type; // Never nullptr, for NULL and UNSET might be empty_type
};
```
and use it where possible instead of `terminal`.
This struct will eventually replace all classes deriving from `terminal`, but for now `terminal` can't be removed completely.
We can't get rid of terminal yet, because sometimes `terminal` is converted back to `term`, which `constant` can't do. This won't be a problem once we replace term with expression.
`bool` is removed from `expression`, now `constant` is used instead.
This is a redesign of PR #9203, there is some discussion about the chosen representation there.
Closes#9244
* github.com:scylladb/scylla:
cql3: term: Remove get_elements and multi_item_terminal from terminals
cql3: Replace most uses of terminal with expr::constant
cql3: expr: Remove repetition from expr::get_elements
cql3: expr: Add expr::get_elements(constant)
cql3: term: remove term::bind_and_get
cql3: Replace all uses of bind_and_get with evaluate_to_raw_view
cql3: expr: Add evaluate_IN_list
cql3: tuples: Implement tuples::in_value::get
cql3: Move data_type to terminal, make get_value_type non-virtual
cql3: user_types: Implement get_value_type in user_types.hh
cql3: tuples: Implement get_value_type in tuples.hh
cql3: maps: Implement get_value_type in maps.hh
cql3: sets: Implement get_value_type in sets.hh
cql3: lists: Implement get_value_type in lists.hh
cql3: constants: Implement get_value_type in constants.hh
cql3: expr: Add expr::evaluate
cql3: values: Add unset value to raw_value_view::make_temporary
cql3: expr: Add constant to expression
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