Clarify immutability (#24)

It took me a few iterations of reading the spec to figure out that bytes
are indeed immutable, and not mutable, as implied by the last note of
HostLoadImportedModule. Similarly, in CreateBytesModule it's not
immediately obvious that the "immutable" descriptor applies to
SharedArrayBuffer as well.

As an unrelated bit of cleanup, the spec metadata is updated to match
the current stage of the proposal.

Author: eemeli

spec.emu

@@ -5,7 +5,7 @@
 <script src="./spec.js"></script>
 <pre class="metadata">
 title: Proposal Import Bytes
-stage: 0
+stage: 2
 contributors: Steven, Guy Bedford
 </pre>
 <emu-intro id="intro">
@@ -75,7 +75,7 @@ contributors: Steven, Guy Bedford
     </emu-note>
 
     <emu-note>
-      <p>All of the import statements in the module graph that address the same JSON <ins>or bytes </ins>module may evaluate to the same mutable object.</p>
+      <p>All of the import statements in the module graph that address the same JSON <ins>or bytes </ins>module may evaluate to the same <del>mutable </del>object.</p>
     </emu-note>
   </emu-clause>
 
@@ -114,7 +114,7 @@ contributors: Steven, Guy Bedford
       <h1>
         <ins>
           CreateBytesModule (
-            _arrayBuffer_: an immutable ArrayBuffer or a SharedArrayBuffer
+            _arrayBuffer_: an immutable ArrayBuffer or an immutable SharedArrayBuffer
           ): a Synthetic Module Record
         </ins>
       </h1>