Add arithmetical commands

Targets #267

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I know that this PR will start a shitstorm, even if I tell you:
Calm down before reviewing this! I didn't do that to piss you
off or something!

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So, here is why I wrote these implementations the way I did:

1. The code in `do_integral_arithmetic()` would be duplicated
four times when writing it the other way.

2. I didn't use macros. Macros would be the alternative to this,
but I know that some of you guys hate large macro foo, and this
would clearly end up beeing a big shitload of macro foo!

3. I wonder how this will get optimized and whether the compiler
is intelligent enough to optimize this into single functions.
Probably it is not, but maybe it does some crazy optimization
here we don't know yet.

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I'm not pissed off or something if this gets rejected, but I
would like to have a calm and premeditated discussion about it,
without bad words and the like!