Make type parameter for future and stream optional

Fixes #65

lang/spec.html

@@ -1603,13 +1603,17 @@ <h5>Visibility</h5>
 <h4>Futures</h4>
 
 <pre
-class="grammar">future-type-descriptor := <code>future</code> type-parameter
+class="grammar">future-type-descriptor := <code>future</code> [type-parameter]
 </pre>
 <p>
 A future value represents a value to be returned by a named worker.  A future
 value belongs to a type <code>future&lt;T></code> if the value to be returned
 belongs to T.
 </p>
+<p>
+A value belongs to a type <code>future</code> (without the type-parameter)
+if it has basic type future.
+</p>
 <h4>[Preview] Services</h4>
 
 <pre
@@ -1632,7 +1636,7 @@ <h4>[Preview] Services</h4>
 <h4>[Preview] Streams</h4>
 
 <pre
-class="grammar">stream-type-descriptor := <code>stream</code> type-parameter
+class="grammar">stream-type-descriptor := <code>stream</code> [type-parameter]
 </pre>
 <p>
 A value of type <code>stream&lt;T></code> is a distributor for values of type
@@ -1641,6 +1645,10 @@ <h4>[Preview] Streams</h4>
 pure type.
 </p>
 <p>
+A value belongs to a type <code>stream</code> (without the type-parameter)
+if it has basic type stream.
+</p>
+<p>
 The implicit initial value of a stream type is a newly created stream, ready to
 distribute values.
 </p>