* Reserve unknown fields in upb
1) For decoding, an unknownfields will be lazily created on message,
which contains bytes of unknown fields.
2) For encoding, if the unknownfields is present on message, all bytes
contained in it will be serialized.

* Register the function to encode unknown field at decode time.

* Remove upb_handlers_setaddunknown

* Use upb_sink_putunknown in decoder

* Remove upb_pb_encoder_encode_unknown

* Do not expose encode_unknown

* Implement reserve unknown field in php Implement.

* Make buffer private to CodedInputStream

* Fix the bug in php c extension that setting one field can change another
field's value.

The reason is that previously, in c extension, it was assumed that the
order that fields were declared in php is the same as the order of
fields in upb. This is not true. Now, for every field in upb, we will
look up the actual property that is corresponding to the upb field.

* Cleanup pull request

* Fix indentation

* Port to php5

* Port with php7.1

* Port to zts

In the generated code of previous versions, each php field is given an
initial value. In c extension, it was assumed that the field order in
the generated code is consistent with upb fields order, so that the
correct initial value can be bound to the correct upb field. However,
this may not be true. The order of fields in generated code is decided
by proto compiler, while the order of upb fields is decided by the hash
function used in c extension.
This PR fixes the issue by reset the initial value at runtime.

* Add json encode/decode for php.

* Fix php conformance test on 32-bit machines.

* Fix conformance test for c extension.

* Fix comments

* Add new file option php_namespace.

Use this option to change the namespace of php generated classes.
Default is empty. When this option is empty, the package name will be
used for determining the namespace.

* Uncomment commented tests

* Revert gdb test change

* Update csharp descriptor.

* Add test for empty php_namespace.

Fix the bug by sharing the code for generating class name for both
message and enum.

This option will be prepended to generated classes of all messages in
the containing file.

This better shows the semantic of the API. For already setted fields,
mergeFromString do replacement for singular fields and appending for
repeated fields.

This method merges the contents of the specified message into the
current message. Singular fields that are set in the specified message
overwrite the corresponding fields in the current message.  Repeated
fields are appended. Map fields key-value pairs are overritten.
Singular/Oneof sub-messages are recursively merged. All overritten
sub-messages are deep-copied.

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

This pull request includes two implementation: C extension and PHP
package. Both implementations support encode/decode of singular,
repeated and map fields.