Fix soundness hole in Ref::into_ref and into_mut (#721) (#726)

This commit implements the fix for #716 which will be released as a new
version in version trains 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7. See #716 for
a description of the soundness hole and an explanation of why this fix
is chosen.

Unfortunately, due to dtolnay/trybuild#241, there is no way for us to
write a UI test that will detect a failure post-monomorphization, which
is when the code implemented in this change is designed to fail. I have
manually verified that unsound uses of these APIs now fail to compile.

Release 0.3.2.

Author: joshlf

Cargo.toml.crates-io renamed to Cargo.toml

@@ -7,7 +7,7 @@
 [package]
 edition = "2018"
 name = "zerocopy"
-version = "0.3.1"
+version = "0.3.2"
 authors = ["Joshua Liebow-Feeser <joshlf@google.com>"]
 description = "Utilities for zero-copy parsing and serialization"
 license = "BSD-3-Clause"