builtins: Add support for variadic arguments to arith functions

This adds support for variadic arguments to PLUS, MINUS, TIMES, QUOTIENT
and REMAINDER. Tests were also modified to reflect the new behaviour,
and a few new cases were added to handle cases that were not possible
before.

This fixes #3.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #9 from algernon/f/vararg-arithmetics

builtins: Add support for variadic arguments to arith functions