* PHP-7.3:
  Fix bug #79465 - use unsigneds as indexes.
  Fix bug #79330 - make all execution modes consistent in rejecting \0

Background
==========
PHP doesn't have any mechanism to inject logic between templating
and final output. There is no way to filter or alter the content
that comes from code in templates like: <?= $someVar ?>

To use php as a robust templataing language, we must inject *some*
logic between templates and their output. We have chosen to make
all <?=, echo, and print statements subject to an optional
trip through escape_html_entitiles.

The functionality can be toggled with `ini_set('__auto_escape')`
and configured with `__auto_escape_flags` and
`__auto_escape_exempt_class` (see commit
2dae5d1 for info about the class
name based auto-escaping exemption.

Methodology
===========
T_ECHO (echo, <?=), T_PRINT (print) now both emit a
ZEND_AST_ECHO_ESCAPE node in the syntax tree.

That's compiled to a function which emits a ZEND_ECHO_ESCAPE op code.

The op code interpretation is a dupe of ZEND_ECHO except with some
if() statements that switch the underlying function from `zend_write`
to `zend_write_escape` based on the ini settings.

zend_write_escape is a new function pointer that points to
php_escape_write.

php_escape_write is a new function that passes it's string argument
through php_escape_html_entities() (with __auto_escape_flags) before
calling the underlying php_output_write.

Use
===
This functionality allows us to safely use php straight as a
templating language with no template compilation step (as many
other templating libraries have).

See the included tests for more usage information.

Exempt Class
============
It is useful to allow some utility functions and helpers to produce
html and have it passed straight through in the template (without
being double-encoded). We accomplish this by *tagging* strings
as being HTML.

    class HtmlString implements JsonSerializable {
       protected $html = '';

       public function __construct($html) {
          $this->html = $html;
       }

       public function __toString() {
          return (string)$this->html;
       }

       public function jsonSerialize() {
          return $this->html;
       }
    }

The auto-escaping system can be configured with an:
__auto_escape_exempt_class="HtmlString"

Which allows instances of `HtmlString` to pass straight through a
template without being modified (skipping the html_entities call).
Helper functions can now return html safely and consumers don't have
to care if it is HTML or not because the auto-escaping system knows
what to do.

squash with big commit

The previous commit updated zend_vm_def and others that are used as
inputs to these auto-generated files.

Run php Zend/zend_vm_gen.php and commit the result.