Divide the decimal128 product by 10^33 or 10^34 with a 128-bit reciprocal

[ibmibmibm]

• mul_finalize_u256 gave the 67 or 68 digit product to
  divmod_pow10_u256. That function does a 512-bit multiply-high, a
  256-bit multiply and a shift loop. A profile showed about 64% of a
  decimal128_t multiply in it.
• 10^k is 5^k * 2^k, with k = 33 or 34. The product shifted right by k bits
  is less than 2^192. A 128-bit reciprocal of 5^k gives the quotient of
  this value or one less, and one compare corrects it.
• The remainder is less than 2 * 5^k, which is less than 2^80, so its low
  128 bits are enough. The low k bits of the product give the rest of the
  remainder.
• Only mul_finalize_u256 changes. divmod_pow10_u256 stays as it is,
  because the constructor and the division also use it.

The results are bit-identical to develop:

• 4 million products for each rounding mode, for decimal128_t and
  decimal_fast128_t. 800,000 of them are exact ties.
• mul_finalize_u256 against the old division on 22 million products,
  with the edges of its input range and the values next to each rounding
  point.
• The same checksums in a -m32 build.

test/benchmarks.cpp, multiplication, median of 6 runs (Ryzen 9 3900X,
GCC 16, -O3):

Type	develop	this PR
decimal128_t	12578813 us	7623474 us
decimal_fast128_t	11266956 us	5837127 us

The checksum s of the benchmark is the same before and after.

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.6%. Comparing base (a7f0eeb) to head (f3bcc02).

Files with missing lines	Patch %	Lines
include/boost/decimal/detail/mul_impl.hpp	96.3%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1466     +/-   ##
=========================================
+ Coverage     98.6%   98.6%   +0.1%     
=========================================
  Files          306     306             
  Lines        25607   25619     +12     
  Branches      2202    2202             
=========================================
+ Hits         25242   25256     +14     
+ Misses         365     363      -2     

Files with missing lines	Coverage Δ
include/boost/decimal/detail/mul_impl.hpp	95.1% <96.3%> (+0.6%)	⬆️

... and 2 files with indirect coverage changes

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[mborland]

Good performance bump. Thanks