Safe transmute between #[repr(transparent)] and the type it contains

[sffc]

I'd like to write the following code:

/// A newtype wrapper around u32 that guarantees the number is even
#[repr(transparent)]
pub struct EvenU32(u32);

impl EvenU32 {
  pub fn try_from_u32_ref(value: &u32) -> Result<&Self, ()> {
    if *value % 2 == 0 {
      Ok(safe_transmute(value))
    } else {
      Err(())
    }
  }
}

To enforce the invariant of the wrapper type, safe_transmute should look at the visibility of the field. For example, the following code in a different module would not work:

// Should not compile, because the `u32` field is inaccessible from the outside
let _: EvenU32 = safe_transmute(&100);

CC @Manishearth