[2/2] Remove some unnecessary MIRI-specific hacks

[ProfFan]

Tested locally with current nightly

[tower120]

I updated CI and fixed cross stable/nightly warnings.

You tried to replace this:

    for i in 0..count {
        *dst.add(i) = *src.add(i);
    }

With this :

ptr::copy(src, dst, count);

Did you benchmark the result? That was done as optimization, because manually copy loop was significantly faster then ptr::copy in my benchmarks in previous compiler versions. I wonder if that was "fixed".

[tower120]

I just did benchmarks. Unfortunately ptr::copy is still very slow on copying small values.