Correct divide-by-zero in cpp_int modulus operations.

[jzmaddock]

Also added some more efficient operator overloads for the trivial case. Fixes https://github.com/boostorg/multiprecision/issues/624

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.1%. Comparing base (f9fdcc2) to head (b0ea443).

:exclamation: Current head b0ea443 differs from pull request most recent head 891e2c3

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