Replace module MIN/MAX and min/max_value() with assoc consts

[faern]

Since it's recommended use use the integer associated constants for new code from Rust 1.43 the docs better mention them instead of the older slightly less recommended versions.

EDIT: Corresponding PR doing the same thing in most of the rust repo comes soon. But I thought submodules get their own PRs(?). Please enlighten me to what the procedure is.

[rust-highfive]

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[faern]

The failing check seems to be spurious, or at least not caused by my changes in this PR.

[Amanieu]

Can you rebase now that #845 is merged?

[faern]

Done