Now that we depend on C++17, we can use these attributes that are part
of the language, instead of relying on macros to make them portable.
(GJS_USE still needs to be defined for public header files, which may be
compiled in C.)

Attributes which are new in C++17 are [[maybe_unused]], [[nodiscard]],
and [[fallthrough]].

[skip cpplint] because cpplint doesn't deal with C++ attributes:
https://github.com/google/styleguide/issues/165

SpiderMonkey 78 will stop using the JSClass.name string as the string
tag. For backwards compatibility, we should make sure the string tag
doesn't change.

See: GNOME/gjs#329

Instead of allocating a separate private struct that allocates a
cairo_surface_t, use the cairo_surface_t directly as the private struct.
This makes the code simpler and saves 8 bytes per object.

With SpiderMonkey 68, it's no longer necessary to mark SpiderMonkey
header files as system headers, and it's also no longer necessary to
include <js-config.h> before any other header files, as SpiderMonkey
has sorted that out internally.

We remove jsapi-wrapper.h and define DEBUG directly in config.h instead
of indirectly in jsapi-wrapper.h via HAVE_SPIDERMONKEY_DEBUG.

This should reduce compile time, on average, because in most .h files
we now only need to include <js/TypeDecls.h>.

Previously #include statements were a bit of a mess across the codebase.
This commit is the result of a pass by the IWYU (Include What You Use)
tool, which suggests headers to add or remove based on what is in the
file, and can also suggest forward-declaring classes instead of
including their headers, if they are only used as a pointer in a
particular file. Cleaning this up should in general speed up compile
times.

IWYU isn't perfect, it produces a number of false positives, so we don't
try to automate this process and we don't accept all of its
recommendations. We do add a script and configuration file to the tools/
directory so that IWYU can be every so often in the future.

We also clean up all the includes according to a consistent style, which
is now described clearly in the C++ style guide.

In files where the IWYU tool would recommend including <stddef.h> only
for the symbol NULL, we instead remove usage of NULL in favour of
C++-style nullptr, or implicit conversion to bool.

This introduces two annotations to functions. GJS_USE is a macro for GCC
and Clang's "warn_unused_result" annotation, meaning that the return
value of a function must not be ignored. This allows us to catch some
cases where we are neglecting to do error checking.

The second annotation is GJS_JSAPI_RETURN_CONVENTION which for now is
identical to GJS_USE, but could be made into a custom annotation in the
future which a static analyzer plugin could use to enforce further checks
at compile time. The "JSAPI return convention" is valid on functions that
return bool or a pointer type, and whose first parameter is JSContext*. If
false or nullptr is returned, then an exception must be pending at return
time on the passed-in JSContext. If true or a non-nullptr value is
returned, then no exception must be pending.

This commit only has the addition of the attributes, and a few fixes
mandated by the autoformatter, so it can be reviewed "lightly".

The return value of g_filename_from_utf8() (must be freed with g_free())
was getting assigned to a GjsAutoJSChar (which would be freed with
JS_free()). Instead gjs_string_to_filename() must have a GjsAutoChar
output parameter.

This requires changing gjs_parse_call_args() as well. We improve it by
allowing to pass in autoptrs for 's' or 'F' args instead of char**.

Fixes a few minor memory leaks, mostly in error paths in Cairo functions.

https://bugzilla.gnome.org/show_bug.cgi?id=785040

Since we moved to a using std::unique_ptr, the filename variables might
be seen by the compiler as possibly uninitialized, so just set them as
mullptr to silence the compiler.

https://bugzilla.gnome.org/show_bug.cgi?id=783031