Add a is_arithmetic() method for types, which can be used to check if
this is a numeric type on which arithmentic operators will allowed -
for example in the following patch to support `SET x = x + 1`.
The arithmetic types are byte, short, int, long, varint, float, double
and decimal.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>