Fix const-correctness of Vector::operator[].

Not sure how this slipped by for so long!

c++/src/kj/array.h

@@ -162,7 +162,11 @@ public:
   }
 
   inline size_t size() const { return size_; }
-  inline T& operator[](size_t index) const {
+  inline T& operator[](size_t index) {
+    KJ_IREQUIRE(index < size_, "Out-of-bounds Array access.");
+    return ptr[index];
+  }
+  inline const T& operator[](size_t index) const {
     KJ_IREQUIRE(index < size_, "Out-of-bounds Array access.");
     return ptr[index];
   }
@@ -356,7 +360,11 @@ public:
 
   inline size_t size() const { return pos - ptr; }
   inline size_t capacity() const { return endPtr - ptr; }
-  inline T& operator[](size_t index) const {
+  inline T& operator[](size_t index) {
+    KJ_IREQUIRE(index < implicitCast<size_t>(pos - ptr), "Out-of-bounds Array access.");
+    return ptr[index];
+  }
+  inline const T& operator[](size_t index) const {
     KJ_IREQUIRE(index < implicitCast<size_t>(pos - ptr), "Out-of-bounds Array access.");
     return ptr[index];
   }

c++/src/kj/vector.h

@@ -52,7 +52,8 @@ public:
   inline size_t size() const { return builder.size(); }
   inline bool empty() const { return size() == 0; }
   inline size_t capacity() const { return builder.capacity(); }
-  inline T& operator[](size_t index) const { return builder[index]; }
+  inline T& operator[](size_t index) { return builder[index]; }
+  inline const T& operator[](size_t index) const { return builder[index]; }
 
   inline const T* begin() const { return builder.begin(); }
   inline const T* end() const { return builder.end(); }