Remove cql3::functions::function_call::raw and replace it with
cql3::expr::function_call, which already existed from the selector
migration to expressions. The virtual functions implementing term::raw
are made free functions and remain in place, to ease migration and
review.
Note that preparing becomes a more complicated as it needs to
account for anonymous functions, which were not representable
in the previous structure (and still cannot be created by the
parser for the term::raw path).
The parser now wraps all its arguments with the term::raw->expr
bridge, since that's what expr::function_call expects, and in
turn wraps the function call with an expr->term::raw bridge, since
that's what the rest of the parser expects. These will disappear
when the migration completes.
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