This deprecates passing null to non-nullable scale arguments of
internal functions, with the eventual goal of making the behavior
consistent with userland functions, where null is never accepted
for non-nullable arguments.

This change is expected to cause quite a lot of fallout. In most
cases, calling code should be adjusted to avoid passing null. In
some cases, PHP should be adjusted to make some function arguments
nullable. I have already fixed a number of functions before landing
this, but feel free to file a bug if you encounter a function that
doesn't accept null, but probably should. (The rule of thumb for
this to be applicable is that the function must have special behavior
for 0 or "", which is distinct from the natural behavior of the
parameter.)

RFC: https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg

Closes GH-6475.

Additionally normalize to using $namespace rather than $uri for
namespace parameters, including in XMLReader and XMLWriter. I went
with that one as it is currently used by DOM, SimpleXML and XSL --
and our DOM parameter names follow the DOM specification.

Closes GH-6295.

Closes GH-6255.

Closes GH-6021

Remove most of the `===DONE===` tags and its variations.
Keep `===DONE===` if the test output otherwise becomes empty.

Closes GH-4872.

The implementation of `XMLReader::open()` and `XMLReader::XML()` still
supports calling the methods statically and non-statically.  However,
as of PHP 8.0.0, calling these methods statically is not allowed,
because they are not declared as static methods.  Since we consider it
to be cleaner to call these methods statically, but had deprecated to
call them statically, we properly support both variants.

We implement support for static and non-static calls by overloading, so
that non-static calls have access to the `$this` pointer.