core: Support building with OpenSSL for checksums

Add an OpenSSL backend to the checksum input stream, which is where we do a lot
of checksumming (object commit, static deltas).

The raw OpenSSL performance is
[approximately double](https://gist.github.com/cgwalters/169349fd1c06fd4fb4d3a7ce33303222) on
my laptop; not only does OpenSSL have e.g. hand-tuned x86_64 assembly, the
current implementation uses the
[Intel SHA extensions](https://en.wikipedia.org/wiki/Intel_SHA_extensions).

Another reason to do this is I was idly thinking about adding
[Curve25519](https://en.wikipedia.org/wiki/Curve25519) signatures (like e.g.
Alpine does) instead of/in addition to GPG.  The rationale for that is
that GPG is pretty heavyweight, both in code footprint and the simple
fact that EC keys are way smaller.

I didn't benchmark ostree with this; we have bigger performance problems
really like the fact we just malloc way too much.  But, it's a step
in the right direction I think in combination with the libcurl work
where we're linking to openssl anyways.

Closes: #738
Approved by: jlebon

src/libostree/ostree-core.c

@@ -1351,16 +1351,7 @@ void
 ostree_checksum_inplace_from_bytes (const guchar *csum,
                                     char         *buf)
 {
-  static const gchar hexchars[] = "0123456789abcdef";
-  guint i, j;
-
-  for (i = 0, j = 0; i < OSTREE_SHA256_DIGEST_LEN; i++, j += 2)
-    {
-      guchar byte = csum[i];
-      buf[j] = hexchars[byte >> 4];
-      buf[j+1] = hexchars[byte & 0xF];
-    }
-  buf[j] = '\0';
+  ot_bin2hex (buf, csum, OSTREE_SHA256_DIGEST_LEN);
 }
 
 /**