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/libotutil/ot-checksum-instream.h

@@ -52,12 +52,6 @@ GType          ot_checksum_instream_get_type     (void) G_GNUC_CONST;
 
 OtChecksumInstream * ot_checksum_instream_new          (GInputStream   *stream, GChecksumType   checksum);
 
-void   ot_checksum_instream_get_digest (OtChecksumInstream *stream,
-                                        guint8          *buffer,
-                                        gsize           *digest_len);
-
-guint8* ot_checksum_instream_dup_digest (OtChecksumInstream *stream,
-                                         gsize              *ret_len);
 char * ot_checksum_instream_get_string (OtChecksumInstream *stream);
 
 G_END_DECLS