swapped formatter

[mefsantos]

I believe the formatter implementation is swapped in HPKEPrivateKey (lines 659 ~ 675)

#[cfg(feature = "hazmat")]
impl std::fmt::Debug for HPKEPrivateKey {
...
     .field("value", &"***")

...


#[cfg(not(feature = "hazmat"))]
impl std::fmt::Debug for HPKEPrivateKey {
...
   .field("value", &self.value)