Document proto2 feature implementations in Google.Protobuf

C# Proto2 feature : Finale

Reworded info about message initialization

Addressed review comments

Java: Add Automatic-Module-Name entries to the Manifest

This commit removes the question template. It also updates the bug report and feature request templates to direct questions to the mailing list.

* When length is zero, substr returns null instead of emptry string, which breaks the invariable for message.
* Tested in https://github.com/protocolbuffers/protobuf/pull/6560

Update version number

* For missing message value, map should create a default message
instance in order to keep its invariable.
* On 32-bit platform, int64 map key should be string

We've notice significant issues downloading from zlib.net. Since there
are already other archives coming from GitHub, this shouldn't negatively
affect reliability.

Optimization for layout_init()

* Add missing files under benchmarks/ to the dist

* fix typo

* Sync upb

* https://github.com/protocolbuffers/upb/pull/208

* Fix php c extension compiling

Don't use macros defined by upb

* Update ruby conformance failure list

Enums use varint, so if the value is negative it should end up being longer.

This was caught my new conformance test cases.

Add comment explaining IExtensionMessage instability

 * Add Syntax enum and make file syntax checks check the enum
 * Made IsPacked a normal get method without ternary op
 * Moved IFieldAccessor.HasValue to IFieldPresenceAccessor
 * Change naming of GetIsExtensionsInitialized
 * Fixed stray text in summary text

Add ExtensionSet tests and add checks that non-message extension fields don't fail initialization checks