Make SettingChange and StreamCharSourcemove constructors and assignment operators noexcept (#808)

The explicitly defaulted or implemented move constructors and assignment
operators are made "noexcept".

Bugfix:
* src/stream.cpp Stream::Stream() char_traits::int_type intro[4] is
  now aggregate-initialized (to zero) to avoid UB.

Minor changes:
* Using std::isinf() and std::signbit() instead of comparing for
  equality with infinity.
* src/streamcharsource.h: Added #include "stream.h".
* src/stream.h: Forward declaring "class StreamCharSource".
* Some implicit casting changed into static_cast's.

Signed-off-by: Ted Lyngmo <ted@lyncon.se>

include/yaml-cpp/emitter.h

@@ -164,13 +164,12 @@ inline Emitter& Emitter::WriteStreamable(T value) {
         std::isnan(value)) {
       special = true;
       stream << ".nan";
-    } else if (std::numeric_limits<T>::has_infinity) {
-      if (value == std::numeric_limits<T>::infinity()) {
-        special = true;
-        stream << ".inf";
-      } else if (value == -std::numeric_limits<T>::infinity()) {
-        special = true;
+    } else if (std::numeric_limits<T>::has_infinity && std::isinf(value)) {
+      special = true;
+      if (std::signbit(value)) {
         stream << "-.inf";
+      } else {
+        stream << ".inf";
       }
     }
   }

include/yaml-cpp/node/detail/impl.h

@@ -33,7 +33,7 @@ struct get_idx<Key,
   static node* get(std::vector<node*>& sequence, const Key& key,
                    shared_memory_holder pMemory) {
     if (key > sequence.size() || (key > 0 && !sequence[key - 1]->is_defined()))
-      return 0;
+      return nullptr;
     if (key == sequence.size())
       sequence.push_back(&pMemory->create_node());
     return sequence[key];