cql3: expr: Add subscript struct
cql3: expr: Add subscript struct

Add a struct called subscript, which will be used in expression
variant to represent subscripted values e.g col[x], val[sub].
It will replace the sub field of column_value.
Having a separate struct in AST for this purpose
is cleaner and allows to express subscripting
values other than column_value.

It is not added to the expression variant yet, because
that would require immediately implementing all visitors.

The following commits will implement individual visitors
and then subscript will finally be added to expression.

Signed-off-by: Jan Ciolek <b66a6f3989e26cb552db14b7ab163c3a50292e24@scylladb.com>

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