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

[gunjjoshi]

Resolves #1988.

Description

What is the purpose of this pull request?

This pull request:

• adds C implementation for math/base/special/gammaln.

double stdlib_base_gammaln( const double x );

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.

Questions

Any questions for reviewers of this pull request?

Shall I change both test.js and test.native.js to use tolerance of the form 1.0 * EPS here? We have a high amount of tolerance required here as well, just like math/base/special/gamma.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

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

[kgryte]

Shall I change both test.js and test.native.js to use tolerance of the form 1.0 * EPS here?

Yes, I think that would be good.

[gunjjoshi]

I've made changes in both C and javascript implementations, according to the latest version, and have updated the links.

[Planeshifter]

Let's land this PR.

I investigated and found that Julia is way closer to our implementation (8.5 * EPS to get tests to pass), so that the large tolerances are due to R using a different algorithm. We should add Julia test fixtures and use them in the unit tests, but that can be done in a follow-up PR.

[gunjjoshi]

Thanks for checking this @Planeshifter! I'll make the changes in a follow-up PR.

[Planeshifter]

@gunjjoshi Thank you!

It may also be useful to add Julia fixtures to gamma, as well. I propose we leave the r fixtures directory intact and just add another one for Julia, and update the test files to use the Julia ones.