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>

src/stream.h

@@ -16,6 +16,9 @@
 #include <string>
 
 namespace YAML {
+
+class StreamCharSource;
+
 class Stream {
  public:
   friend class StreamCharSource;