Make substr not have an optional argument

[masak]

Right now substr takes an optional third argument. Which means that both 2 and 3 arguments work with calling that function. It's not a big thing, but it's slightly annoying, because nothing else in 007 works that way.

We could go any of the following ways in resolving that:

1. Make the third parameter non-optional. Live with the consequences: having to write out the third argument always, even when we want the rest of the string.
2. Split up substr into two functions. Maybe call them substr(s, pos, chars) and suffix(s, pos). For consistency, maybe also add a prefix(s, pos).
3. Add optional parameters (via Make it possible to switch on optional parameters/parameter defaults/rest parameters #112, but the first step is enough). Declare that the setting as a compilation unit makes use of the optional_parameters pragma.

I'm kind of leaning towards the second option right now because it's easier. (Maybe there is prior art somewhere containing better names than prefix and suffix? It has to be two words that mean "start" and "end"; using "left" and "right" is unfair to RTL writing systems.) But really, any solution on the list is acceptable.