Add simplified readonly type

Equivalent to union of inherently immutable basic types.
No type parameter.
Fixes #462. First step of #189.

lang/spec.html

@@ -544,7 +544,7 @@ <h4>Type descriptors</h4>
    <td>a sequence of values that can be generated lazily</td>
   </tr>
   <tr>
-   <td rowspan="8">other</td>
+   <td rowspan="9">other</td>
    <td>singleton</td>
    <td>a single value described by a literal</td>
   </tr>
@@ -577,6 +577,10 @@ <h4>Type descriptors</h4>
    <td>the union of (), int, float, decimal, string, and maps and arrays whose
 values are, recursively, json</td>
   </tr>
+  <tr>
+   <td>readonly</td>
+   <td>the union of all inherently immutable basic types</td>
+  </tr>
 </table>
 <p>
 In addition to describing a type, a type descriptor may also include information
@@ -2113,6 +2117,7 @@ <h3>Other type descriptors</h3>
    | never-type-descriptor
    | byte-type-descriptor
    | json-type-descriptor
+   | readonly-type-descriptor
    | type-reference
    | <code>(</code> type-descriptor <code>)</code>
 </pre>
@@ -2242,6 +2247,35 @@ <h4>JSON types</h4>
 </p>
 
 </section>
+<section>
+<h4>Readonly type</h4>
+<pre
+class="grammar">readonly-type-descriptor := <code>readonly</code>
+</pre>
+<p>
+The <code>readonly</code> type is a predefined name for the union of all basic types
+that are inherently immutable. These are:
+</p>
+<ul>
+<li>all simple types
+<ul>
+<li>nil</li>
+<li>boolean</li>
+<li>int</li>
+<li>float</li>
+<li>decimal</li>
+</ul>
+</li>
+<li>string</li>
+<li>error</li>
+<li>function</li>
+<li>service</li>
+<li>typedesc</li>
+<li>handle</li>
+</ul>
+
+</section>
+
 <section>
 <h4>Type reference</h4>
 <pre
@@ -7550,6 +7584,7 @@ <h2 id="changes">B. Changes since previous versions</h2>
 <section>
 <h3>Summary of changes from 2020R1 to 2020R2</h3>
 <ol>
+<li>The <code>readonly</code> type has been added.</li>
 <li>The XML item-specific subtypes can be used as constructors, both in
 expressions and in binding patterns.</li>
 </ol>