Round a short decimal128_t product near the smallest exponent

[ibmibmibm]

• The decimal128_t multiply has a separate path for an exponent sum below -6142. It does not
  expand the significands, so the product can have 1 to 68 digits.
• The path rounded the product only above 38 digits, the digits10 of the 128-bit significand
  type. A product of 35 to 38 digits then went to pack_in_range unrounded. The encoder wrote
  its extra digits into bits outside the significand field.
• The limit is now detail::precision_v<ReturnType>, 34 digits for decimal128_t.
• The new test github_issue_1482.cpp multiplies products of 35 to 38 digits in each rounding
  mode. Before the change, each check fails except one product that rounds to zero.

A sweep of 20000 such products gives 962 wrong results in each of the five rounding modes on
develop, and none after the change. The change needs #1477, which is in develop. Issue
#1482 gives the values, the sweep, the comparison with develop and the benchmark.

Fixes #1482

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.8%. Comparing base (c4e5174) to head (edb1ee2).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1483     +/-   ##
=========================================
+ Coverage     98.7%   98.8%   +0.1%     
=========================================
  Files          315     316      +1     
  Lines        26449   26473     +24     
  Branches      2253    2253             
=========================================
+ Hits         26104   26129     +25     
+ Misses         345     344      -1     

Files with missing lines	Coverage Δ
include/boost/decimal/detail/mul_impl.hpp	95.2% <100.0%> (ø)
test/github_issue_1482.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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