[RFC] remove vim_round() in favour of round()

C89 did not have round(), vim emulated it with vim_round. But since we're using C99 this is not a problem anymore.

This conflicts a tiny bit with the profiling PR (#839), I'll rebase whichever one gets merged second.
It's not really about exploits, but me and @oni-link were wondering if this change in behaviour for signed 0 is important/relevant. As posted by @oni-link:

```c
echo string(round(0))."123" // used to be -0.0123, with this patch it is 0.0123
```