some TOC reorganization

As I go through this, I'm finding some ways that I want to tweak the order.

Author: steveklabnik

src/doc/trpl/SUMMARY.md

@@ -20,19 +20,19 @@
     * [Primitive Types](primitive-types.md)
     * [Functions](functions.md)
     * [Comments](comments.md)
-    * [Structs](structs.md)
-    * [Mutability](mutability.md)
-    * [Method Syntax](method-syntax.md)
-    * [Enums](enums.md)
     * [`if`](if.md)
-    * [Match](match.md)
-    * [Patterns](patterns.md)
     * [`for` loops](for-loops.md)
     * [`while` loops](while-loops.md)
     * [Ownership](ownership.md)
     * [References and Borrowing](references-and-borrowing.md)
     * [Lifetimes](lifetimes.md)
+    * [Mutability](mutability.md)
     * [Move semantics](move-semantics.md)
+    * [Enums](enums.md)
+    * [Match](match.md)
+    * [Patterns](patterns.md)
+    * [Structs](structs.md)
+    * [Method Syntax](method-syntax.md)
     * [Drop](drop.md)
     * [Vectors](vectors.md)
     * [Arrays](arrays.md)