cql3: grammar: collapse conversions between term::raw and expressions
cql3: grammar: collapse conversions between term::raw and expressions

The grammar now talks to expression API:s solely, so it can be converted
internally to expressions too. Calls to as_term_raw() and as_expression()
are removed, and productions return expressions instead of term::raw:s.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla