Fix compatibility with latest nightly toolchain version

[crabm4n]

Platform intrinsics were removed and are replaced with core_intrinsics (now accessible via std::intrinsics::simd).

This is a breaking change as the simd_llvm and simd_traits which were publicly exported are removed.

[xMAC94x]

Hi @yoanlcq this MR is necessary for the veloren team to update the rust toolchain and newer toolschains had become incompatible with vek . do you have some time for a review?
best regards,
Marcel

[yoanlcq]

Hey guy, sorry for the delay! It's been a long 5-day week-end here in France.

This looks good to me. I was about to ask whether there was any way to be backwards compatible with prior compiler versions, but I guess there's no point: SIMD was always unstable and required explicit opt-in by using Nightly and enabling the platform_intrinsics feature. Plus, previous versions of vek are always available.

Thank you @crabm4n for your effort, I very much appreciate it.
You have waited long enough, so I'll merge this right now, then push a new version. If you'd like to add yourself to the list of contributors (README + Cargo.toml) then feel free to do so even in a separate PR.

[crabm4n]

Thank you and no worries for the delay ;D

[yoanlcq]

This is now available in version 0.17.0 along with many warning fixes.

[xMAC94x]

Thank you very much. No worries, we all have a personal live that matter first. Waiting a few days is absolutely no issue :)
Hope you enjoyed the long and sunny weekend :)