Use unsafe_op_in_unsafe_fn

RFC #2585 has been updated to be an optional lint for now. We clearly want this
behavior, so use this lint for our code. While here, also enable some other
lints that sound useful.

We are actually missing a couple of unsafe{} blocks in unsafe functions, so
follow-up commits are needed to properly document the safety assumptions around
these.

See: rust-lang/rfcs#2585 (comment)
See: rust-lang/rust#71668

src/lib.rs

@@ -1,6 +1,11 @@
 #![warn(missing_docs)]
-#![warn(rustdoc::missing_doc_code_examples)]
+#![warn(nonstandard_style)]
 #![warn(rust_2018_idioms)]
+#![warn(rustdoc::missing_doc_code_examples)]
+#![warn(single_use_lifetimes)]
+#![warn(unsafe_op_in_unsafe_fn)]
+#![warn(unused_crate_dependencies)]
+#![warn(unused)]
 
 //! Allocate heap memory with user-specified alignment.
 
@@ -189,7 +194,6 @@ impl<T> AlignedBox<[T]> {
     // # SAFETY
     // The initializer function has to initialize the value behind the pointer without
     // reading or dropping the old uninitialized value, e.g., using std::ptr::write.
-    #[allow(unused_unsafe)] // https://github.com/rust-lang/rfcs/pull/2585
     unsafe fn new_slice(
         alignment: usize,
         nelems: usize,
@@ -232,7 +236,6 @@ impl<T> AlignedBox<[T]> {
     // # SAFETY
     // The initializer function has to initialize the value behind the pointer without
     // reading or dropping the old uninitialized value, e.g., using std::ptr::write.
-    #[allow(unused_unsafe)] // https://github.com/rust-lang/rfcs/pull/2585
     unsafe fn realloc(
         &mut self,
         nelems: usize,