#3190 round/floor/ceiling slow for hi-precision bfloats

[Stavros Macrakis]

fpprec:1000000$
b1: bfloat(1) ... 1 ms
floor(b1) ... 2 ms
ceiling(b1) ... 16480 ms
round(b1) ... 24938 ms

I believe an equally accurate solution can be found much more efficiently using

round(rationalize(b1)) ... 7 ms

[Stavros Macrakis]

Actually, the speedup is much smaller in the general case:

fpprec:200000$
b11:1.0b0+1.0b-199999$
round(b11) ... 1276 ms
round(rationalize(b11)) ... 279 ms

fpprec:1000000$
b3: 1.0b0/3$
round(b3) ... 32035 ms
round(rationalize(b3)) ... 9173 ms

... and for "floor" and "ceiling", the existing method can be faster:

floor(b3) ... 3 ms
floor(rationalize(b3)) ... 3288 ms;
ceiling(b3) ... 16379 ms
ceiling(rationalize(b3)) ... 3318 ms

so this may require some additional analysis.