SmallSet: Mark iterator parent as const (#4613)

We already assume the parent does not change,

binaryen/src/support/small_set.h

Lines 202 to 208 in 94d77ef
 // std::set allows changes while iterating. For us here, though, it would 
 // be nontrivial to support that given we have two iterators that we 
 // generalize over (switching "in the middle" would not be easy or fast), 
 // so error on that. 
 if (usingFixed != other.usingFixed) { 
   Fatal() << "SmallSet does not support changes while iterating"; 
 } 

This also marks the parent as const to reflect that.

This fixed a weird C++ compilation error I had when working on
something unrelated, but seems worth landing independently.

Author: kripken

src/support/small_set.h

@@ -165,7 +165,7 @@ template<typename T, size_t N, typename FlexibleSet> class SmallSetBase {
     using pointer = const value_type*;
     using reference = const value_type&;
 
-    Parent* parent;
+    const Parent* parent;
 
     using Iterator = IteratorBase<Parent, FlexibleIterator>;
 
@@ -176,7 +176,7 @@ template<typename T, size_t N, typename FlexibleSet> class SmallSetBase {
     size_t fixedIndex;
     FlexibleIterator flexibleIterator;
 
-    IteratorBase(Parent* parent)
+    IteratorBase(const Parent* parent)
       : parent(parent), usingFixed(parent->usingFixed()) {}
 
     void setBegin() {