syntax: functions require return type. remove ->

The purpose of this is:

 * Only one way to do things
 * Changing a function with void return type to return a possible
   error becomes a 1 character change, subtly encouraging
   people to use errors.

See #632

Here are some imperfect sed commands for performing this update:

remove arrow:

```
sed -i 's/\(\bfn\b.*\)-> /\1/g' $(find . -name "*.zig")
```

add void:

```
sed -i 's/\(\bfn\b.*\))\s*{/\1) void {/g' $(find ../ -name "*.zig")
```

Some cleanup may be necessary, but this should do the bulk of the work.

src/ast_render.cpp

@@ -450,10 +450,9 @@ static void render_node_extra(AstRender *ar, AstNode *node, bool grouped) {
                 }
 
                 AstNode *return_type_node = node->data.fn_proto.return_type;
-                if (return_type_node != nullptr) {
-                    fprintf(ar->f, " -> ");
-                    render_node_grouped(ar, return_type_node);
-                }
+                assert(return_type_node != nullptr);
+                fprintf(ar->f, " ");
+                render_node_grouped(ar, return_type_node);
                 break;
             }
         case NodeTypeFnDef: