* Add conformance test for nested listvalue

* Fix upb for parsing repeated Value/ListValue

* Add failed repeated ListValue conformance test into php failure list

json_decode automatically convert numeric string to integer, so that
we need to convert it back. However, this will suceed to parse invalid
json data with string field set as integer even though it should have failed.
Because, the failure case is less often than the succeeding case, we decided
to make this change. Users should make sure their data don't use integer for
string fields by themselves.

* Fix php well known type conformance tests

* Properly generate code for test.proto

* Provide GPBMetadata files in c extensions for generated files to import.

* Remove unnecessary test

* Clean up code

* Add declaration for initOnce.

* Refactoring

* Fix php well known type conformance tests

* Properly generate code for test.proto

* Provide GPBMetadata files in c extensions for generated files to import.

* Remove unnecessary test

* Clean up code

* Add declaration for initOnce.

* Refactoring

* Add json encode/decode for php.

* Fix php conformance test on 32-bit machines.

* Fix conformance test for c extension.

* Fix comments