Make Float work with no_std (rebased #50)

[cuviper]

• Use ::core::num::FpCategory when deriving Float to work with no_std
• Get rid of unnecessary double-borrowing to fix clippy lint

[cuviper]

This is a rebase of #50 without the inlining change that I was not comfortable with.

[cuviper]

bors r+

[bors[bot]]

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