refactor: Remove ForEach discovery, cache MethodInfo

Remove ForEach type argument collection from source generator.
Cache MethodInfo for RemoveComponentImpl in World.cs.
Update API docs for IRelationship and ChangeKind.

docs/api-surface.md

@@ -379,7 +379,6 @@ namespace OECS;
 
 public interface IRelationship
 {
-    Entity Source { get; }
     Entity Target { get; }
 }
 ```
@@ -399,8 +398,7 @@ namespace OECS;
 public struct Relationship<TSelf, TTarget> : IRelationship
     where TSelf : struct where TTarget : struct
 {
-    [Key(0)] public Entity Source { get; set; }
-    [Key(1)] public Entity Target { get; set; }
+    [Key(0)] public Entity Target { get; set; }
 }
 ```
 
@@ -424,7 +422,8 @@ public enum ChangeKind
     EntityRemoved,
     ComponentAdded,
     ComponentRemoved,
-    ComponentModified
+    ComponentModified,
+    RelationshipReordered
 }
 ```