Make checksum_impl.h safe to compile with -fstrict-aliasing.

In general, Postgres requires -fno-strict-aliasing with compilers that
implement C99 strict aliasing rules.  There's little hope of getting
rid of that overall.  But it seems like it would be a good idea if
storage/checksum_impl.h in particular didn't depend on it, because
that header is explicitly intended to be included by external programs.
We don't have a lot of control over the compiler switches that an
external program might use, as shown by Michael Banck's report of
failure in a privately-modified version of pg_verify_checksums.

Hence, switch to using a union in place of willy-nilly pointer casting
inside this file.  I think this makes the code a bit more readable
anyway.

checksum_impl.h hasn't changed since it was introduced in 9.3,
so back-patch all the way.

Discussion: https://postgr.es/m/1535618100.1286.3.camel@credativ.de

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