Coercion bypasses DistributedSlice typecheck

[CAD97]

Reproduction:

#[distributed_slice]
pub static SLICE: [&'static str];

#[distributed_slice(SLICE)]
static ELEMENT1: &&str = &"uhoh";

#[distributed_slice(SLICE)]
static ELEMENT2: &&str = &"ohno";

fn main() {
    eprintln!("{:?}", SLICE[0].as_bytes().as_ptr_range());
    // Possible output: 0x7ff75a903c88..0xffeeb5207928
    // That's 140 TB of memory just available to read
}

Culprit: DistributedSlice::private_typecheck(self, T) accepts an argument of type &&str and coerces it to &str.