Enable clippy::expect_used, address warnings

This enables Clippy's
[`expect_used`](https://rust-lang.github.io/rust-clippy/master/index.html#/expect_used)
lint, which errors on calls to `Option::expect` and `Result::{expect,
expect_err}`. The intent of enabling this lint is to reduce the number
of panics emitted in `zerocopy`'s code (issue #202).

src/lib.rs

@@ -187,6 +187,7 @@
     clippy::correctness,
     clippy::dbg_macro,
     clippy::decimal_literal_representation,
+    clippy::expect_used,
     clippy::get_unwrap,
     clippy::indexing_slicing,
     clippy::missing_inline_in_public_items,
@@ -225,6 +226,7 @@
     // production code introduce the possibly of code panicking unexpectedly "in
     // the field".
     clippy::arithmetic_side_effects,
+    clippy::expect_used,
     clippy::indexing_slicing,
 ))]
 #![cfg_attr(not(test), no_std)]
@@ -1550,6 +1552,7 @@ pub unsafe trait FromZeros {
     /// * Panics if allocation of `size_of::<Self>() * len` bytes fails.
     #[cfg(feature = "alloc")]
     #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
+    #[allow(clippy::expect_used)]
     #[inline]
     fn new_box_slice_zeroed(len: usize) -> Box<[Self]>
     where
@@ -2992,10 +2995,7 @@ pub unsafe trait AsBytes {
         Self: NoCell,
     {
         let start = bytes.len().checked_sub(mem::size_of_val(self))?;
-        bytes
-            .get_mut(start..)
-            .expect("`start` should be in-bounds of `bytes`")
-            .copy_from_slice(self.as_bytes());
+        bytes.get_mut(start..)?.copy_from_slice(self.as_bytes());
         Some(())
     }
 }
@@ -4350,6 +4350,7 @@ where
     /// `new_slice` panics if `T` is a zero-sized type.
     #[inline]
     pub fn new_slice(bytes: B) -> Option<Ref<B, [T]>> {
+        #[allow(clippy::expect_used)]
         let remainder = bytes
             .len()
             .checked_rem(mem::size_of::<T>())
@@ -4874,6 +4875,7 @@ where
         // PANICS: By invariant, `self.0` satisfies `T`'s alignment and is a
         // valid size for `T`, which ensures that `try_cast_into_no_leftover`
         // will not panic.
+        #[allow(clippy::expect_used)]
         Ptr::from(self.0)
             .try_cast_into_no_leftover::<T>()
             .expect("Ref::deref_unchecked: internal error (pointer cast should never fail)")