[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.
I saw it. I can say that the only place where I would expect this difference to matter are tests. But I almost never use floats in VimL, so know almost no related features and hacks.

Can show you a Fortran floating-point number parser in a rather free format I once written that is almost (but really not) F77. It is hacky. Surprising how rarely I use floats in programs, and how fast they are converted to something else (normally for displaying) and forgotten: my fortran program is the last thing I remember that was built around floats.