Support thread locals

[Nadrieril]

This code:

thread_local!(static FOO: u32 = 0);
fn main() {}

Causes this error when translating with charon:

error[E9999]: Unexpected type `ty` for scalar `scalar`

              Context:
               - ty: Binder { value: fn(std::option::Option<&'^0.Named(std::thread::local::{impl#4}::new::'_, "'_") mut std::option::Option<u32>>) -> *const u32, bound_vars: [Region(BrNamed(std::thread::local::{impl#4}::new::'_, '_))] }
               - scalar: alloc4
  |
  = note: ⚠️ This is a bug in Hax's frontend.
          Please report this error to https://github.com/hacspec/hax/issues with some context (e.g. the current crate)!

warning: Hax panicked when translating `Body { ... }`