+---------------------+
             |     BaseObject      |
             +---------------------+
                        |
                        |
                        |
             +---------------------+
             |   NativeKeyObject   |
             +---------------------+
                        |
                        |
                        |
             +---------------------+
             |      KeyObject      |
             +---------------------+
               /                 \
              /                   \
             /                     \
            /                       \
+---------------------+    +---------------------+
|   SecretKeyObject   |    | AsymmetricKeyObject |
+---------------------+    +---------------------+
                             /                 \
                            /                   \
                           /                     \
                          /                       \
              +---------------------+   +---------------------+
              |   PublicKeyObject   |   |   PrivateKeyObject  |
              +---------------------+   +---------------------+

This separates key handles from the actual key data:

+-----------------+
| NativeKeyObject |
+-----------------+
        ^
     extends
        |
+-----------------+    +-----------------+    +---------------+
| KeyObject  (JS) | -> | KeyObjectHandle | -> | KeyObjectData |
+-----------------+    +-----------------+    +---------------+

This change allows sharing KeyObjects between threads via postMessage.
The receiver acquires a new KeyObject and a new KeyObjectHandle, but
refers to the same KeyObjectData:

+-------------------+
| NativeKeyObject 1 | ------------------------------------------+
+-------------------+                                           |
        ^                                                       |
     extends                                                    |
        |                                                       |
+-------------------+    +-------------------+                  |
| KeyObject 1  (JS) | -> | KeyObjectHandle 1 | --------------+  |
+-------------------+    +-------------------+               |  |
                                                             |  |
                                                             |  |
                                                             |  |
                                                             |  |
                                                             |  |
+-------------------+                                        |  |
| NativeKeyObject 2 | ------------------------------------+  |  |
+-------------------+                                     |  |  |
        ^                                                 |  |  |
     extends                                              |  |  |
        |                                                 |  |  |
+-------------------+    +-------------------+            |  |  |
| KeyObject 2  (JS) | -> | KeyObjectHandle 2 | --------+  |  |  |
+-------------------+    +-------------------+         |  |  |  |
                                                       |  |  |  |
                                                       |  |  |  |
                                                       |  |  |  |
                                                       |  |  |  |
                                                       |  |  |  |
+-------------------+                                  |  |  |  |
| NativeKeyObject 3 | ------------------------------+  |  |  |  |
+-------------------+                               |  |  |  |  |
        ^                                           |  |  |  |  |
     extends                                        |  |  |  |  |
        |                                           v  v  v  v  v
+-------------------+    +-------------------+    +---------------+
| KeyObject 3  (JS) | -> | KeyObjectHandle 3 | -> | KeyObjectData |
+-------------------+    +-------------------+    +---------------+

Co-authored-by: Anna Henningsen <anna@addaleax.net>