Move non_send_fields_in_send_ty to suspicious in Rust 1.58

[xFrednet]

Clippy #7709 introduced non_send_fields_in_send_ty. It was decided to first keep it in the nursery group and then elevate it to warn-by-default in the next version (Rust 1.58). The goal is to get some early feedback.

I'll assign myself to this issue to not lose track of it. Anyone else can still create an PR for this in the next version 🙃. I'm also interested in developing a better mechanism for cases like this, but that's a story for another time ^^

cc: @Qwaz

[xFrednet]

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