Handle very basic column type conversions in SST3
Handle very basic column type conversions in SST3

After this change very simple schema changes of column type
will work. This change makes sure that variable size and fixed
size types can be converted to each other but only if their bit
representation can be automatically converted between those types.

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

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