Bump deps/libtommath from `4929f09` to `42b3fb0`

[dependabot[bot]]

Bumps deps/libtommath from 4929f09 to 42b3fb0.

Commits

• 42b3fb0 Merge pull request #577 from diizzyy/patch-1
• a2fe83f cmake: Remove unnecessary ccache logic
• a9a11d2 Merge pull request #538 from libtom/small-stacksize
• 3570e12 Disable MP_SMALL_STACK_SIZE on MSVC
• 6c70ef1 Run small-stack tests with valgrind
• 4c2e177 Use appveyor build matrix
• 513d48d regen
• 33a9d0d Use Thread Local Storage for warray buffer
• 334465d Update makefiles
• fae9aa5 Add tests for MSVC multi-threading
• Additional commits viewable in compare view

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