embassy-net: when std is set enable smoltcp/std

[dpeckett]

As explained in the title, this is mostly to solve some clippy issues with ManagedSlice's I've been running into.

[Dirbaio]

nothing in embassy-net requires smoltcp apis that are only available under std, so it's not possible that this fixes any issues.

The problem with smoltcp+managed you're encountering is typically due to something enabling managed/std or managed/alloc behind smoltcp's back so they get out of sync. Check which crate it is with cargo tree --format '{p} {f}' and remove it, and you'll find everything builds.

[dpeckett]

Thanks @Dirbaio, you were of course right. the managed crate was getting pulled in elsewhere.