Update AsPrimitive Safety docs for rust 1.45

[martin-t]

Fixes #174

The safety section should not be removed entirely since people might be using older compilers (as was pointed out to me in a related issue in the vek crate).

However, this and followup comments indicate that the second case (float to float) was never UB in the first place - should the second example be removed?

[cuviper]

Hi,

However, this and followup comments indicate that the second case (float to float) was never UB in the first place - should the second example be removed?

Yes, I think we can remove that altogether -- would you like to do that here?

[martin-t]

Sure, I also changed the phrasing into past tense in the first example and removed the bullet point since it's no longer a list. Is is ok like this?

[cuviper]

Works for me, thanks!

bors r+

[bors]

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