Teach Refs.Add() to append to the reflog
Introduce TreeEntry.TargetType and TreeEntryDefinition.TargetType
Obsolete TreeEntry.Type and TreeEntryDefinition.Type
Introduce ObjectType
Deploy ObjectType to OdbBackend

Sadly, this is a breaking change as there's no way to allow a migration path through the use of an [Obsolete] attribute.
Obsolete GitObjectType
Obsolete Tree.Trees and Tree.Blobs properties

Relying on this properties would trigger multiple enumerations of the
Tree entries. Exposing them is quite misleading for the consumer.
Update libgit2 binaries to b641c00

https://github.com/libgit2/libgit2/compare/08283cb...b641c00
Fix documentation