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>
df20854963