There's no benefit to using C include guards so switch to pragma once
everywhere for consistency.
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Also replace derived types (map_type, collection_type, etc.).
As we'll change data_type's definition, this reduces the number of places
that need to be modified later, and is more readable.
In Origin, some places in CQL3 package treat ColumnDefinition as
ColumnSpecification. Origin solves that by making ColumnDefinition
extend ColumnSpecification.
I find it much simpler to provide an external adapter, which is cached
as a field for efficiency. Another, more important, reason for this
solution is that column_specifications are passed around as
shared_ptrs and I don't want to add unnecessary indirection to
column_definition accesses just because of that, to make it inherit
from column_specification.
