[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.
You're right. Arguably this was a bit of an implementation error of `vim_round()` (or not, signed 0 is tricky business anyway). The question is, indeed, whether anyone was relying on this.

As the master of seemingly impossible vimscript features that people are using, I'd like to call on @ZyX-I.