feat: add stats/base/dists/wald/cdf

[Neerajpathak07]

Progresses #209.

Description

What is the purpose of this pull request?

This pull request:

• Adds stats/base/dists/wald/cdf

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• RFC: add the Wald distribution #209

Questions

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No.

Other

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No.

Checklist

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• Read, understood, and followed the contributing guidelines

AI Assistance

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• Test/benchmark generation
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{{TODO: add disclosure if applicable}}

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/wald/cdf	$\color{red}326/334$ $\color{red}-0.59%$	$\color{red}27/31$ $\color{red}-2.85%$	$\color{green}4/4$ $\color{green}+0.00%$	$\color{red}326/334$ $\color{red}-0.59%$

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

[Neerajpathak07]

Given that the CI error is not due to changes in this PR will go ahead and open it for review!!

[Neerajpathak07]

Representing phi in this format ref:-

[Neerajpathak07]

Could have used stats/base/dists/normal/cdf but while writing the C implementation using normal_cdf into the equation causes discrepancy in the return values.

[Neerajpathak07]

Throws an error if 0.998 is rounded of to 1.000 so used this instead!!

[Planeshifter]

Left a bunch of comments. Thanks!

[Neerajpathak07]

@Planeshifter Updated the changes. But, benchmarks do seem to fail but runs clean locally.
Is it a bug in this PR??