core/sugar: defstruct and defrecord seal by default

ober

61251c19419a12d2b3b800bc1a09f51f2cde6404

diff --git a/lib/jerboa/core.sls b/lib/jerboa/core.sls
index c6109f6..30d70e9 100644
--- a/lib/jerboa/core.sls
+++ b/lib/jerboa/core.sls
@@ -400,6 +400,7 @@
                                             (gensym (symbol->string name-sym)))])
                  #'(begin
                      (define-record-type (hidden-name mid pid)
+                       (sealed #t)
                        (fields (mutable field iacc imut) ...))
                      (define tid (record-type-descriptor hidden-name))
                      (define acc iacc) ...
diff --git a/lib/std/sugar.sls b/lib/std/sugar.sls
index 923b9ef..92ff3c0 100644
--- a/lib/std/sugar.sls
+++ b/lib/std/sugar.sls
@@ -418,6 +418,7 @@
       [(_ name (field ...))
        (begin
          (define-record-type name
+           (sealed #t)
            (fields field ...))
          (defrecord-extras name (field ...)))]))