Add leading_ones and trailing_ones to PrimInt

[clarfonthey]

Since this was only stabilised in 1.46.0, it falls back to a naïve version on older versions.

It seems unlikely that the version in libstd will ever be different from calling the zeros versions on !self, but for future-proofing, this defers to the libstd versions anyway.

[clarfonthey]

Rebased and fixed issue mentioned

[clarfonthey]

ping @cuviper, I think you'd be the one to merge this?

[cuviper]

bors r+

[bors[bot]]

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