docs: rename WrenValue to WrenHandle across embedding docs and C API

Rename the `WrenValue` type to `WrenHandle` in the public C header, update all
function signatures (`wrenMakeCallHandle`, `wrenCall`, `wrenReleaseHandle`),
and rename the documentation page from "Slots and Values" to "Slots and Handles"
with corresponding link updates throughout the embedding guide and module source
files.

doc/site/embedding/template.html

@@ -24,7 +24,7 @@
       <h2>embedding</h2>
       <ul>
         <li><a href="./">Introduction</a></li>
-        <li><a href="slots-and-values.html">Slots and Values</a></li>
+        <li><a href="slots-and-handles.html">Slots and Handles</a></li>
         <li><a href="calling-wren-from-c.html">Calling Wren from C</a></li>
         <li><a href="calling-c-from-wren.html">Calling C from Wren</a></li>
         <li><a href="storing-c-data.html">Storing C Data</a></li>
@@ -44,7 +44,7 @@
         <td>
           <ul>
             <li><a href="./">Introduction</a></li>
-            <li><a href="slots-and-values.html">Slots and Values</a></li>
+            <li><a href="slots-and-handles.html">Slots and Handles</a></li>
             <li><a href="calling-wren-from-c.html">Calling Wren from C</a></li>
             <li><a href="calling-c-from-wren.html">Calling C from Wren</a></li>
             <li><a href="storing-c-data.html">Storing C Data</a></li>