New MD API: rename functions from _ext to _ret

The _ext suffix suggests "new arguments", but the new functions have
the same arguments. Use _ret instead, to convey that the difference is
that the new functions return a value.
diff --git a/programs/hash/hello.c b/programs/hash/hello.c
index a0c08c7..2e8c224 100644
--- a/programs/hash/hello.c
+++ b/programs/hash/hello.c
@@ -54,7 +54,7 @@
 
     mbedtls_printf( "\n  MD5('%s') = ", str );
 
-    if( ( ret = mbedtls_md5_ext( (unsigned char *) str, 13, digest ) ) != 0 )
+    if( ( ret = mbedtls_md5_ret( (unsigned char *) str, 13, digest ) ) != 0 )
         return( MBEDTLS_EXIT_FAILURE );
 
     for( i = 0; i < 16; i++ )