df20854963
Move the classes representing CQL expressions (and utility functions on them) from the `restrictions` namespace to a new namespace `expr`. Most of the restriction.hh content was moved verbatim to expression.hh. Similarly, all expression-related code was moved from statement_restrictions.cc verbatim to expression.cc. As suggested in #5763 feedback https://github.com/scylladb/scylla/pull/5763#discussion_r443210498 Tests: dev (unit) Signed-off-by: Dejan Mircevski <dejan@scylladb.com>