[mycpp/runtime refactor] Make Dict Slab policy more like the List one

It's simplified, because all the key, value, and index Slabs should all
be the same size.

mycpp/gc_dict.h

@@ -37,6 +37,11 @@ List<T>* ListFromDictSlab(Slab<int>* index, Slab<T>* slab, int n) {
 
 template <class K, class V>
 class Dict {
+  // Relates to minimum slab size.  This is good for Dict<K*, V*>, Dict<K*,
+  // int>, Dict<int, V*>, but possibly suboptimal for Dict<int, int>.  But that
+  // case is rare.
+  static const int kMinItems = 4;
+
  public:
   Dict()
       : GC_CLASS_FIXED(header_, field_mask(), sizeof(Dict)),
@@ -121,6 +126,14 @@ class Dict {
   }
 
   DISALLOW_COPY_AND_ASSIGN(Dict)
+
+ private:
+  int RoundCapacity(int n) {
+    if (n < kMinItems) {
+      return kMinItems;
+    }
+    return RoundUp(n);
+  }
 };
 
 template <typename K, typename V>
@@ -157,7 +170,7 @@ void Dict<K, V>::reserve(int n) {
   //
   if (capacity_ < n) {  // TODO: use load factor, not exact fit
     // calculate the number of keys and values we should have
-    capacity_ = RoundUp(n + kCapacityAdjust) - kCapacityAdjust;
+    capacity_ = RoundCapacity(n + kCapacityAdjust) - kCapacityAdjust;
 
     // TODO: This is SPARSE.  How to compute a size that ensures a decent
     // load factor?

mycpp/gc_list.h

@@ -25,18 +25,13 @@ class GlobalList {
 
 template <typename T>
 class List {
-  // TODO: Move methods that don't allocate or resize: out of gc_heap?
-  // - allocate: append(), extend()
-  // - resize: pop(), clear()
-  // - neither: reverse(), sort() -- these are more like functions.  Except
-  //   sort() is a templated method that depends on type param T.
-  // - neither: index(), slice()
-
+  // Relate slab size to number of items (capacity)
   // 8 / 4 = 2 items, or 8 / 8 = 1 item
   static const int kCapacityAdjust = kSlabHeaderSize / sizeof(T);
   static_assert(kSlabHeaderSize % sizeof(T) == 0,
                 "Slab header size should be multiple of item size");
 
+  // Relates to minimum Slab size.
   // Smallest non-empty List<T*>  should have about 4 items, or 3 without header
   // Smallest non-empty List<int> should have about 8 items, or 7 without header
   static const int kMinItems = 32 / sizeof(T);

mycpp/gc_slab.h

@@ -6,18 +6,14 @@
 #include "mycpp/common.h"  // DISALLOW_COPY_AND_ASSIGN
 #include "mycpp/gc_obj.h"  // GC_OBJ
 
-// Return the size of a resizeable allocation.  For now we just round up by
-// powers of 2. This could be optimized later.  CPython has an interesting
-// policy in listobject.c.
+// Return the size of a resizeable allocation.  Just round up to the nearest
+// power of 2.  (CPython has an interesting policy in listobject.c.)
 //
 // https://stackoverflow.com/questions/466204/rounding-up-to-next-power-of-2
-inline int RoundUp(int n) {
-  // Note: List::RoundCapacity refines this.  TODO: remove this check when we
-  // have Dict::RoundCapacity.
-  if (n < 4) {
-    return 4;
-  }
+//
+// Used by List<T> and Dict<K, V>.
 
+inline int RoundUp(int n) {
   // TODO: what if int isn't 32 bits?
   n--;
   n |= n >> 1;