feat: add C implementation for math/base/special/modf

[gunjjoshi]

Resolves #1916.

Description

What is the purpose of this pull request?

This pull request:

• adds C implementation for math/base/special/modf

void stdlib_base_modf( const double x, double* integral, double* frac )

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves a part of [RFC]: Add C implementations to base special math functions (tracking issue) #649.
• fixes [RFC]: add C implementation for math/base/special/modf #1916.

Questions

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No.

Other

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No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• ☑️ Read, understood, and followed the contributing guidelines.

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@stdlib-js/reviewers

[Pranavchiku]

Ahh, tests fail, @gunjjoshi can you please iterate over implementation again and see where the flaw is, something is off somewhere. Try it, else ping us, will take a close look.

....
      ✖ returns integral part
      ✖ returns fractional part


  total:     10042
  passing:   6036
  failing:   4006
  duration:  391ms

[gunjjoshi]

Hey @Pranavchiku, all tests pass now. There was an issue after the last commit. Also, I've made changes as suggested. Thanks for pointing them out !

[Pranavchiku]

LGTM. Thanks @gunjjoshi !