lib: Add a lighter weight internal checksum wrapper

The faster (OpenSSL/GnuTLS) code lived in a `GInputStream` wrapper, and that
adds a lot of weight (GObject + vtable calls). Move it into a simple
autoptr-struct wrapper, and use it in the metadata path, so we're
now using the faster checksums there too.

This also drops a malloc there as the new API does hexdigest in place to a
buffer.

Prep for more work in the commit path to avoid `GInputStream` for local file
commits, and ["adopting" files](https://github.com/ostreedev/ostree/pull/1255).

Closes: #1256
Approved by: jlebon

src/libostree/ostree-core-private.h

@@ -20,6 +20,7 @@
 #pragma once
 
 #include "ostree-core.h"
+#include "otutil.h"
 #include <sys/stat.h>
 
 G_BEGIN_DECLS
@@ -77,7 +78,7 @@ gboolean _ostree_write_variant_with_size (GOutputStream      *output,
                                           GVariant           *variant,
                                           guint64             alignment_offset,
                                           gsize              *out_bytes_written,
-                                          GChecksum          *checksum,
+                                          OtChecksum         *checksum,
                                           GCancellable       *cancellable,
                                           GError            **error);