Added basic detection of `noexcept` support for some compilers, added
corresponding RAPIDJSON_NOEXCEPT annotations to
* non-allocating constructors
* (move) assignment
* Swap
With the new string handling API, the constructor taking a `bool`
parameter matches in some unwanted cases, as pointers can be casted
to `bool` implicitly.
Add a SFINAE helper to this constructor to avoid matching arbitrary
pointers. To avoid confusion for the user, this mechanism is hidden
from the Doxygen documentation.
