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

[gunjjoshi]

Description

What is the purpose of this pull request?

This pull request:

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

double stdlib_base_digamma( 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.

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No.

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• Read, understood, and followed the contributing guidelines.

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@stdlib-js/reviewers

[Planeshifter]

Implementation looks fine and matches what we have in JS. Thanks for the PR, @gunjjoshi!

However, it seems that the Boost reference implementation has changed.

Version this is derived from:
https://www.boost.org/doc/libs/1_53_0/boost/math/special_functions/digamma.hpp

Latest version:
https://www.boost.org/doc/libs/1_85_0/boost/math/special_functions/digamma.hpp

Would you mind looking into updating the implementation to track what is in Boost in a follow-up PR?

[gunjjoshi]

Yes @Planeshifter, I'll do that.