Make md_info_t an opaque structure

- more freedom for us to change it in the future
- enforces hygiene
- performance impact of making accessors no longer inline should really be
  negligible
diff --git a/ChangeLog b/ChangeLog
index b0900d2..ac04f82 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,7 @@
   * Support for DTLS 1.0 and 1.2 (RFC 6347).
 
 API Changes
+   * Change md_info_t into an opaque structure (use md_get_xxx() accessors).
    * Some constness fixes
    * Signature of mpi_mul_mpi() changed to make the last argument unsigned
    * Remove the PBKDF2 module (use PKCS5).