use a ContiguousMap instead of a stringmap -> HiddenClass

when looking up a child hidden class we look up the attr in the child_index map,
then get the HiddenClass* from the children vector (at that index.)

this speeds up GC substantially by letting us visit the range from the vector as
opposed to iterating through the stringmap.

src/gc/collector.cpp

@@ -191,6 +191,9 @@ bool GCVisitor::isValid(void* p) {
 }
 
 void GCVisitor::visit(void* p) {
+    if (!p)
+        return;
+
     if (isNonheapRoot(p)) {
         return;
     } else {

src/runtime/objmodel.cpp

@@ -474,7 +474,7 @@ HiddenClass* HiddenClass::getOrMakeChild(const std::string& attr) {
 
     auto it = children.find(attr);
     if (it != children.end())
-        return it->second;
+        return children.getMapped(it->second);
 
     static StatCounter num_hclses("num_hidden_classes");
     num_hclses.log();

src/runtime/types.h

@@ -22,6 +22,7 @@
 #include "structmember.h"
 
 #include "codegen/irgen/future.h"
+#include "core/contiguous_map.h"
 #include "core/threading.h"
 #include "core/types.h"
 #include "gc/gc_alloc.h"
@@ -286,7 +287,7 @@ private:
 
     // Only makes sense for NORMAL hidden classes.  Clients should access through getAttrOffsets():
     llvm::StringMap<int> attr_offsets;
-    llvm::StringMap<HiddenClass*> children;
+    ContiguousMap<llvm::StringRef, HiddenClass*, llvm::StringMap<int>> children;
 
 public:
     static HiddenClass* makeRoot() {
@@ -308,11 +309,8 @@ public:
 
     void gc_visit(GCVisitor* visitor) {
         // Visit children even for the dict-backed case, since children will just be empty
-        for (const auto& p : children) {
-            visitor->visit(p.second);
+        visitor->visitRange((void* const*)&children.vector()[0], (void* const*)&children.vector()[children.size()]);
     }
-    }
-
 
     // Only makes sense for NORMAL hidden classes:
     const llvm::StringMap<int>& getAttrOffsets() {