Add constant MAX_NEGATIVE to each floating-point value

[SciMind2460]

Since the constants fxx::MAX and fxx:MIN mirror each other, I thought it would be a good idea to add constants that fxx::MIN_POSITIVE mirrors. Hence, I have added: fxx::MAX_NEGATIVE.

#116909 is a related issue.

[rustbot]

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[workingjubilee]

Please do not have commits with messages like "Update f32.rs". Describe what is actually done.

[joboet]

r? libs-api

[tgross35]

How could this constant be stable since 1.43 when you are adding it now? :)

New API needs an unstable gate first, and is moved to stable later.

[tgross35]

API changes need an API change proposal (ACP), which is an issue template at https://github.com/rust-lang/libs-team. You'll have to justify the use case a bit.

Personally I'm hesitant to add more repr-specific (i.e. non-mathematical) constants until we discuss something about the confusing names of the constants we do have, cc #88734.

[SciMind2460]

@tgross35 Fair enough. I thought that given that there is already MIN_POSITIVE, MAX_NEGATIVE added more symmetry between both constants.