[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.
`round(0)` and `vim_round(0)` have different signs. Is a use case like ths
```vimL
echo string(round(0))."123" // 0.0123 or -0.0123
```
relevant?