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/ostree-core.c

@@ -1272,6 +1272,20 @@ ostree_checksum_to_bytes_v (const char *checksum)
   return ot_gvariant_new_bytearray ((guchar*)result, OSTREE_SHA256_DIGEST_LEN);
 }
 
+/**
+ * ostree_checksum_b64_to_bytes:
+ * @checksum: An ASCII checksum
+ *
+ * Returns: (transfer full) (array fixed-size=32): Binary version of @checksum.
+ */
+guchar *
+ostree_checksum_b64_to_bytes (const char *checksum)
+{
+  guchar *ret = g_malloc (32);
+  ostree_checksum_b64_inplace_to_bytes (checksum, ret);
+  return ret;
+}
+
 /**
  * ostree_checksum_inplace_from_bytes: (skip)
  * @csum: (array fixed-size=32): An binary checksum of length 32
@@ -1363,6 +1377,23 @@ ostree_checksum_from_bytes_v (GVariant *csum_v)
   return ostree_checksum_from_bytes (ostree_checksum_bytes_peek (csum_v));
 }
 
+/**
+ * ostree_checksum_b64_from_bytes:
+ * @csum: (array fixed-size=32): An binary checksum of length 32
+ *
+ * Returns: (transfer full): Modified base64 encoding of @csum
+ *
+ * The "modified" term refers to the fact that instead of '/', the '_'
+ * character is used.
+ */
+char *
+ostree_checksum_b64_from_bytes (const guchar *csum)
+{
+  char *ret = g_malloc (44);
+  ostree_checksum_b64_inplace_from_bytes (csum, ret);
+  return ret;
+}
+
 /**
  * ostree_checksum_bytes_peek:
  * @bytes: #GVariant of type ay