Move generated test code for Google.Protobuf.Test to a separate lib

if the value was >32bit with the low bits all being zero, it was decoded as
false instead of as true.

Fixes #6863

Bring .NET Framework target down to 4.5
Rename project in comment

Set lang version to C# 3.0
Add .NET Standard 2.0 target
Fix partial diagnostic string since it requires C# 6

Change _Extensions property to normal body rather than expression

Ruby lazy wrappers optimization

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.)