The implementation is non-standard. Deprecation allows a simpler API in
the future without removing any features.

NaN values never compare equal to themselves or to other NaN values.

https://blog.travis-ci.com/2017-03-28-xcode-83-ga-is-here

We now only demand our exceptions to be is_nothrow_copy_constructible
if std::runtime_exception is.

The solution with a std::runtime_error member is more elegant. It
allows to have std::exception as base class again. However, I still
have no idea why GCC thinks the copy constructor may throw...

To have nothrow-copy-constructible exceptions, we inherit from
std::runtime_error which can cope with arbitrary-length error messages.
Intermediate strings are built with static functions and then passed to
the actual constructor.

Add Visual Studio 17 image to appveyor build matrix

This commit removed the re2c lexer and replaced it by a manual version.
Its integration is not yet complete: number parsing does not respect
locales or overflows. Furthermore, parsing does not need to end with
EOF. Therefore, a lot of test cases fail. The idea is to push this
branch forward so we can conduct performance comparisons. So far, a
nice side effect are better diagnosis messages in case of parse errors.