* Inherit from message instead of implement

When regestering class (implmenets other class) during MINIT, zend_class_implements
would call zend_class_entry->interface_gets_implemented(). In PHP-7.3 interface_gets_implemented shares the same location with create_object.
However, during MINIT, the global object storeage hasn't been initialized. And thus, caused segment fault in php 7.3.

* Use zend_string_init where interned string may be the value.
zend_string_dup will keep using the existing interned string.
In php 7.3, interned string cannot be destroyed from user's code.

* Uncommment debug code

* Use latest phpunit for each php versions

* Revert change in Dockerfile

* Update php test to use the new docker image

* Update composer

* Change docker organization

* Update phpunit

* Debug phpunit

* Store phpunit into bin dir in docker image

* Install valgrind to docker

* Fix compatibility test

* Remove generated_service_test from compatibility c extension test

* Update 32bit php test to the new docker image

* Install bison

* Fix build.sh

* Fix DOCKERIMAGE_PREFIX

* Fix basename

* Add comment to build_and_run_docker2.sh

* Remove commented code

* Fix comments

* adds string-to-int and int-to-string methods to enums

* remove check for valueToName property in EnumTrait

* Remove unused imports

* Update to avoid using EnumTrait

* Remove EnumTrait

* Update enum types

* Move name and value methods into generated classes

* Remove functions from GPBUtil

* Test well known enums

* Implement enum value to/from name in c extension

* Only generate use statement when namespace is present

* Fix well known type class not inheriting Message

Previously, well known types only implement Message interface but not inhert it.
Fix the problem by calling zend_do_inheritance

* Add back commented tests

* php: Added nanosecond support for Timestamp

* php: Fixed compatibility test

* uses namespaces for nested messages and enums

* fixes namespaces for PHP dist

* fixes namespace for Descriptors, adds Cardinality and Kind

* fixes nested namespaces for reserved words and adds tests

* adds tests and generator fix for php class prefixes

* fixes escaping of protobuf packages, enum comments, misc others

* nice refactor of generated code

* adds class files for backwards compatibility

* simplifies code with templates

* adds compatibility files to makefile

* cleanup of generator and fixes nested namespace bug

* regenerates proto types

* remove internal BC classes

* adds deprecated warning, adds methods back

* simplifies if statement

* fixes dist files

* addresses review comments

* adds back TYPE_URL_PREFIX constant

* adds @deprecated to old nested class files

* skips tests which require a separate process when protobuf.so is enabled

* Adds tests for legacy nested classes that do not require separate processes to test

* uses legacy names for GPBUtil message check

* adds block for IDE @deprecated message

* Namespace for nested message/enum in c extension

* Remove unused code

* uses namespaces for nested messages and enums

* fixes namespaces for PHP dist

* fixes namespace for Descriptors, adds Cardinality and Kind

* fixes nested namespaces for reserved words and adds tests

* adds tests and generator fix for php class prefixes

* fixes escaping of protobuf packages, enum comments, misc others

* nice refactor of generated code

* adds class files for backwards compatibility

* simplifies code with templates

* adds compatibility files to makefile

* cleanup of generator and fixes nested namespace bug

* regenerates proto types

* remove internal BC classes

* adds deprecated warning, adds methods back

* simplifies if statement

* fixes dist files

* addresses review comments

* adds back TYPE_URL_PREFIX constant

* adds @deprecated to old nested class files

* skips tests which require a separate process when protobuf.so is enabled

* Adds tests for legacy nested classes that do not require separate processes to test

* uses legacy names for GPBUtil message check

* adds block for IDE @deprecated message

* Namespace for nested message/enum in c extension

* Remove unused code

* Add well known types to php runtime.

* Fix php7.0 tests

* No longer generate empty.proto in test as it has been included in
runtime.

* Fix zts build

* Clean code

* Rename g_p_b_empty to empty.

* Don't generate code for empty.proto in compatibility test

* Fix 32-bit

* Fix mac build

* Fix Makefile.am to add new files

* Add php support for Timestamp.

* Fix comments

* Add any support in php runtime.

* Remove unused file in config.m4

* Fix comments

* Fix error for tsrmls build

* Add newly added file to Makefile.am

descriptor.proto uses proto2 syntax, which is not ready for external
usage. However, some proto3 files import descriptor.proto and cannot be
used. In this PR, all references (We cheated by only removing
extensions, which is enough for now. User should avoid using messages
defined in descriptor.proto as field type.) to content in
descriptor.proto are removed from generated files. Those that import
descriptor.proto can be used like other proto files.

1. Generate single file for each message.
2. Lazily initiate metadata.

In php, class name cannot be "Empty". Modified code generator to (#2375) generate GPBEmpty instead (for google.protobuf.Empty only). Also change
runtime code to work with the new generated code accordingly.