- <js/Array.h> and <js/ValueArray.h> are new headers.
- Rename JS_NewArrayObject to JS::NewArrayObject.
- Rename JS_GetArrayLength to JS::GetArrayLength.
- Rename JS_IsArrayObject to JS::IsArrayObject.
- Rename JS::AutoValueArray to JS::RootedValueArray.

See: GNOME/gjs#329

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

This allows to automatically use the proper union value depending on the
type without having to do it manually.

(Philip: changed some formatting, added char32_t for GI_TYPE_TAG_UNICHAR
and changed gjs_arg_value() to gjs_arg_get() if the union member wasn't
needed)

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_t,
use the cairo_t directly as the private struct. This makes the code
simpler and saves 8 bytes per object. The only thing to watch out for is
that the pointer can be null if it was already disposed.

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>.

This should save 16-24 bytes of memory per Cairo object, easy win!

Previously, passing the wrong kind of object where a Cairo.Path,
Cairo.Pattern (subclass), or Cairo.Surface (subclass) was expected,
would crash. This adds type checking to avoid these crashes.

The normal way to do this would be with JS_HasInstance(), but since
Cairo.Pattern and Cairo.Surface are abstract classes, that won't work.
JS_HasInstance() takes a constructor, and abstract classes don't have a
constructor. Instead, we have to check the passed-in object's prototype
chain. It turns out there isn't a JSAPI function to do that, so we add
gjs_object_in_prototype_chain() for this purpose.

In addition we add missing error checking in a few places.

Closes: #49.

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.