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

[Neerajpathak07]

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Resolves None.

Description

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This pull request:

• Adds stats/base/dists/wald/logcdf.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/wald/logcdf	$\\color{green}363/363$ $\\color{green}+0.00\\%$	$\\color{green}35/35$ $\\color{green}+0.00\\%$	$\\color{green}4/4$ $\\color{green}+0.00\\%$	$\\color{green}363/363$ $\\color{green}+0.00\\%$

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

[Neerajpathak07]

For this package the fixtures for testing were generated in the following manner. The reason being I went with this was that while using

logcdf( InverseGaussian( mu[i], lambda[i] ), x[i] ) );

The fixtures generated had higher diffs and even while testing the ULP diffs would naturally exceed 1500.

[kgryte]

In general, we shouldn't be "cooking" the tests to get the results we want. If there are meaningful discrepancies with equivalent APIs in reference implementations, the answer is to investigate and understand why the discrepancies exist in the first place. Some may be legitimate; others may encourage us to update our implementation. But simply seeking out the answer that we want in order to make the tests "look better" is never--never--the answer. That simply is not the stdlib way.

[Planeshifter]

Looks good to me after some clean-up.

Most notably, the implementation was tested against log(cdf(x)) of Julia instead of its dedicated logcdf function, masking numerical issue in the right tail where the sum ≈ 1 gives unbounded relative error as logCDF → 0⁻.

The high ULP difference is consistent with what we have for normal/logcdf after regenerating the fixtures and updating the implementation.