Standardize our use of begin/end iterators

We try to keep our use of C++ dead simple for non-C++ programmers to
read easily, but there are cases where we need to yield a bit. (For
example, using shared pointers to keep memory management simple.)

In order to use the std::sort() function, we need to provide or use
begin and end iterators. With the recent change to our
`estimate_halfway.cc` program, we had different ways of using such
iterators.

The BVH code used the member functions `begin()` and `end()`, while the
`estimate_halfway` program used argument promotion from an array and the
array plus an integer offset.

To standardize both and reduce variance, this change uses the
currently-recommended practice of using `std::begin(thing)` and
`std::end(thing)`.

For now, I'm avoiding spending any text explaining to the non-C++ reader
what these are, in the hopes that the meaning can be easily deduced.

Author: hollasch

src/TheNextWeek/bvh.h

@@ -49,7 +49,7 @@ class bvh_node : public hittable {
             left = objects[start];
             right = objects[start+1];
         } else {
-            std::sort(objects.begin() + start, objects.begin() + end, comparator);
+            std::sort(std::begin(objects) + start, std::begin(objects) + end, comparator);
 
             auto mid = start + object_span/2;
             left = make_shared<bvh_node>(objects, start, mid);

src/TheRestOfYourLife/bvh.h

@@ -49,7 +49,7 @@ class bvh_node : public hittable {
             left = objects[start];
             right = objects[start+1];
         } else {
-            std::sort(objects.begin() + start, objects.begin() + end, comparator);
+            std::sort(std::begin(objects) + start, std::begin(objects) + end, comparator);
 
             auto mid = start + object_span/2;
             left = make_shared<bvh_node>(objects, start, mid);

src/TheRestOfYourLife/estimate_halfway.cc

@@ -46,7 +46,7 @@ int main() {
     }
 
     // Sort the samples by x.
-    std::sort(samples, samples + N, compare_by_x);
+    std::sort(std::begin(samples), std::end(samples), compare_by_x);
 
     // Find out the sample at which we have half of our area.
     double half_sum = sum / 2.0;