Dear all,

we need to check that the operands of arithmetic binary operations
are consistent and of numeric type.

The PR reported an issue for multiplication ("*"), but we better
extend this to the other binary operations.

I chose the following solution:
- consistent types for +,-,*,/, keeping an internal error if any
  unhandled type shows up,
- numeric types for **

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

Thanks,
Harald