feat: add C implementation for math/base/special/falling-factorial

[gunjjoshi]

Progresses #649.

Description

What is the purpose of this pull request?

This pull request:

• adds C implementation for math/base/special/falling-factorial.

double stdlib_base_falling_factorial( const double x, const int32_t n )

Related Issues

Does this pull request have any related issues?

This pull request:

• progresses [RFC]: Add C implementations to base special math functions (tracking issue) #649.

Questions

Any questions for reviewers of this pull request?

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

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/falling-factorial	$\color{red}298/322$ $\color{green}+92.55\%$	$\color{red}30/35$ $\color{green}+85.71\%$	$\color{green}3/3$ $\color{green}+100.00\%$	$\color{red}298/322$ $\color{green}+92.55\%$

The above coverage report was generated for the changes in this PR.

[gunjjoshi]

/stdlib update-copyright-years

[Planeshifter]

Didn't see any problems, so approving this PR. Thanks, @gunjjoshi!