Add kj::zeroTo(x), equivalent to kj::range(0, x).

The problem with kj::range() is that if the inputs are not recognized as the same type, the typechecker can get annoyed.

c++/src/kj/common.h

@@ -643,6 +643,7 @@ template <typename T>
 class Range {
 public:
   inline constexpr Range(const T& begin, const T& end): begin_(begin), end_(end) {}
+  inline explicit constexpr Range(const T& end): begin_(0), end_(end) {}
 
   class Iterator {
   public:
@@ -690,6 +691,14 @@ inline constexpr Range<Decay<T>> range(T begin, T end) { return Range<Decay<T>>(
 //     // Prints 1, 2, 3, 4, 5, 6, 7, 8, 9.
 //     for (int i: kj::range(1, 10)) { print(i); }
 
+template <typename T>
+inline constexpr Range<Decay<T>> zeroTo(T end) { return Range<Decay<T>>(end); }
+// Returns a fake iterable container containing all values of T from zero (inclusive) to `end`
+// (exclusive).  Example:
+//
+//     // Prints 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
+//     for (int i: kj::zeroTo(10)) { print(i); }
+
 template <typename T>
 inline constexpr Range<size_t> indices(T&& container) {
   // Shortcut for iterating over the indices of a container: