core: Add allocating b64 checksum functions

The checksum_b64_inplace variants can't be used in bindings. Provide
versions that allocate and return the output rather than working on a
passed in buffer. These can then be used in GI bindings to get the
ostree modified base64 encodings.

Closes: #398
Approved by: cgwalters

src/libostree/libostree.sym

@@ -351,9 +351,8 @@ global:
  *                         NOTE NOTE NOTE
  */
 
-/* UNCOMMENT WHEN ADDING THE FIRST NEW SYMBOL FOR 2016.8
 LIBOSTREE_2016.8 {
 global:
-	insert_symbol_here;
+        ostree_checksum_b64_to_bytes;
+        ostree_checksum_b64_from_bytes;
 } LIBOSTREE_2016.7;
-*/