core: unstable::global::get_global_state: Rewrite more of the runtime in Rust

[catamorphism]

I was surprised there wasn't already an umbrella issue for this... or maybe I'm not seeing it.

[pnkfelix]

The bug here, as I understand it, is that get_global_state is using some unsafe pointer manipulation with an atomic_cxchg to install a task-global lookup table.

I do not see why this is assigned to maturity milestone #2: backwards compatibility.

It strikes me as just an enhancement to the code base. Tagging as I-nominated, and suggest we remove milestone tag entirely.

[graydon]

code in question is already deleted