Implement FromZeroes for thin raw pointers

[joshlf]

Makes progress on #170

[jswrenn]

Is it clearly documented anywhere that 0 is a valid value for the null pointer? Infamously, the C standard does not require that null pointers have an all-zero bit-pattern, only that they compare as equal to 0.

[jswrenn]

This snippet from the internals of the standard library gives me pause:

pub const fn invalid<T>(addr: usize) -> *const T {
    // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
    // We use transmute rather than a cast so tools like Miri can tell that this
    // is *not* the same as from_exposed_addr.
    // SAFETY: every valid integer is also a valid pointer (as long as you don't dereference that
    // pointer).
    unsafe { mem::transmute(addr) }
}

The use of mem::transmute suggests that these sorts of transmutes are fine now, but what about the comment "I am magic and should be a compiler intrinsic"?

[joshlf]

Current plan is to wait until this change (or something similar) lands so the soundness of this PR is clearly backed by the Rust reference.

[joshlf]

Closing in favor of #584.