C#: Optimize parsing of some primitive and wrapper types

Use 3 parameter Encoding.GetString for default string values

C#: Add microbenchmark for parsing wrapper types

Fix Makefile.am by adding generated files

Fix reflection access when using old generated code

Avoid NullReferenceException when accessing CustomOptions

Make shared libraries be able to link to MSVC static runtime libraries, so that VC runtime is not required.

Even though the property is deprecated, it still shouldn't throw.

FileDescriptor construction uses an extension registry including extensions from imports. If these were created using an older version of protoc, the FieldDescriptor.Extension property may be null; we ignore such extensions rather than failing.

(The implementation to fix this is in the following commit.)

* Avoid initializing primitive fields in layout_init

* Avoid initializing string/bytes/message fields in layout_init

* Lazily create map when needed

* Lazily create repeated fields

* Change layout_init to only do memcpy

* Fix test for php-7.0

* Fix conformance test where default value of string/message map is not encoded

* Fix test for zts

* Clean up

* Fix comments

Update GSON and errorprone

Update to 3.10.0 in docs

This way, we can avoid an unnecessary copy.

merge

Down integrate Google3 changes to Github