pull out for loop body of rounding test for later test
add explicit test cases for rounding corner cases
fix rounding errors

 - the direct predecessor/successor for 0.5/-0.5 was formerly wrongly
   rounded to 1.0/-1.0 rather than 0.0

 - integral numbers of the format 1xxxxxx1 filling mantissa completely
   were formerly rounded to the next integral away from zero
Add extended tests for difficult cases from https://github.com/boostorg/math/pull/8
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