#25 OP_ROUND: Use nearbyint

[Jesse Rosenstock]

R5RS (and later) rounds to nearest even for compatibility with the
default IEEE-754 rounding mode.  nearbyint() uses the current rounding
mode, will still be round-t0-nearest-even unless someone has called
fesetround().  roundeven() from TS 18661 ignores the rounding mode and
would be ideal, but it's not widely implemented as of 2020.