Tests assembly made friend.

This allows tests to access internal classes inside Migrant assembly.
Typestamp related classes made internal.
Merge branch 'master' of https://github.com/antmicro/Migrant into HEAD

Conflicts:
	Migrant/AssemblyInfo.cs
	Migrant/VersionTolerance/FieldDescriptor.cs
	Migrant/VersionTolerance/TypeDescriptor.cs
	Migrant/VersionTolerance/TypeStamp.cs