* 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.