Reduce truncation in dec_float multiplication

[ckormanyos]

The purpose of this PR is to reduce truncation in cpp_dec_float's multiplication. It is based on some discussion in parts of #368.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e584f4f) 94.1% compared to head (73fb4d3) 94.1%.

Additional details and impacted files

[](https://app.codecov.io/gh/boostorg/multiprecision/pull/585?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ```diff @@ Coverage Diff @@ ## develop #585 +/- ## ======================================= Coverage 94.1% 94.1% ======================================= Files 273 273 Lines 28523 28523 ======================================= Hits 26835 26835 Misses 1688 1688 ``` | [Files](https://app.codecov.io/gh/boostorg/multiprecision/pull/585?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) | Coverage Δ | | |---|---|---| | [include/boost/multiprecision/cpp\_dec\_float.hpp](https://app.codecov.io/gh/boostorg/multiprecision/pull/585?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-aW5jbHVkZS9ib29zdC9tdWx0aXByZWNpc2lvbi9jcHBfZGVjX2Zsb2F0LmhwcA==) | `94.0% <100.0%> (ø)` | | ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/boostorg/multiprecision/pull/585?src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/boostorg/multiprecision/pull/585?src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Last update [e584f4f...73fb4d3](https://app.codecov.io/gh/boostorg/multiprecision/pull/585?src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg).

[ckormanyos]

Hi Matt (@mborland) I can configure the CodeCov Deltas. I'm a bit surprised that my trivial change led to such a decrease. I'll look into this.

Anyway, we dont want fails due to a small one, two percent delta. I can handle this on the weekend in the CodeCov Properties for this workspace.