Warning in docs of num_traits::cast::AsPrimitive seems to be out-of-date

[jesperdj]

The documentation of num_traits::cast::AsPrimitive warns about undefined behavior:

Currently, some uses of the as operator are not entirely safe. In particular, it is undefined behavior if:
...
Or a floating point value does not fit in another floating point type (#15536)

However, issue 15536 is closed and fixed, as far as I understand, and this is not undefined behavior anymore (in fact, it was never really undefined behavior; the LLVM docs described it as such, but those docs have been clarified).

It looks like the warning in the documentation of AsPrimitive is not accurate anymore.

[Enet4]

That is good news! Resolving this issue appears to be a simple as removing the "Safety" section.

[cuviper]

The float as float issue was deemed safe -- we can remove that.

But up through Rust 1.44, float as int is still UB if it overflows. In Rust 1.45, that operation will instead be defined to saturate, so only then it will be safe. We should probably still clarify this situation in the AsPrimitive docs.

Semi-related, #162 started using to_int_unchecked() in cases where we've already checked the boundary conditions, to avoid any possible performance hit of the upcoming saturation.