Additional work towards 1.16 compatibility

- Very basic implementation of the SideEffects system. Will definitely need fine tuning for it to be functional, but is not considered a priority in my opinion.
- Minor changes to the World interface and World implementations related to the SideEffects system. Shouldn't be the cause of any new bugs but be on the lookout.
- Included debug in BukkitImplLoader.java to assist contributors in understanding what needs to be implemented for the adapter to load properly.

Still very WIP but we're a few steps closer. So far, this is coming along better than I anticipated. Hopefully we can keep the momentum.

worldedit-core/src/main/java/com/sk89q/worldedit/LocalSession.java

@@ -72,6 +72,7 @@ import com.sk89q.worldedit.session.ClipboardHolder;
 import com.sk89q.worldedit.util.Countable;
 import com.sk89q.worldedit.util.HandSide;
 import com.sk89q.worldedit.util.Identifiable;
+import com.sk89q.worldedit.util.SideEffectSet;
 import com.sk89q.worldedit.world.World;
 import com.sk89q.worldedit.world.block.BaseBlock;
 import com.sk89q.worldedit.world.block.BlockState;
@@ -145,7 +146,7 @@ public class LocalSession implements TextureHolder {
     private transient Snapshot snapshotExperimental;
     private transient boolean hasCUISupport = false;
     private transient int cuiVersion = -1;
-    private transient boolean fastMode = false;
+    private transient SideEffectSet sideEffectSet = SideEffectSet.defaults();
     private transient Mask mask;
     private transient Mask sourceMask;
     private transient TextureUtil texture;
@@ -1494,7 +1495,7 @@ public class LocalSession implements TextureHolder {
             builder.blockBag(blockBag);
         }
         builder.command(command);
-        builder.fastmode(fastMode);
+        builder.fastmode(!this.sideEffectSet.doesApplyAny());
 
         editSession = builder.build();
 
@@ -1513,7 +1514,7 @@ public class LocalSession implements TextureHolder {
     }
 
     private void prepareEditingExtents(EditSession editSession, Actor actor) {
-        editSession.setFastMode(fastMode);
+        editSession.setSideEffectApplier(sideEffectSet);
         editSession.setReorderMode(reorderMode);
         if (editSession.getSurvivalExtent() != null) {
             editSession.getSurvivalExtent().setStripNbt(!actor.hasPermission("worldedit.setnbt"));
@@ -1521,13 +1522,32 @@ public class LocalSession implements TextureHolder {
         editSession.setTickingWatchdog(tickingWatchdog);
     }
 
+    /**
+     * Gets the side effect applier of this session.
+     *
+     * @return the side effect applier
+     */
+    public SideEffectSet getSideEffectSet() {
+        return this.sideEffectSet;
+    }
+
+    /**
+     * Sets the side effect applier for this session
+     *
+     * @param sideEffectSet the side effect applier
+     */
+    public void setSideEffectSet(SideEffectSet sideEffectSet) {
+        this.sideEffectSet = sideEffectSet;
+    }
+
     /**
      * Checks if the session has fast mode enabled.
      *
      * @return true if fast mode is enabled
      */
+    @Deprecated
     public boolean hasFastMode() {
-        return fastMode;
+        return !this.sideEffectSet.doesApplyAny();
     }
 
     /**
@@ -1535,8 +1555,9 @@ public class LocalSession implements TextureHolder {
      *
      * @param fastMode true if fast mode is enabled
      */
+    @Deprecated
     public void setFastMode(boolean fastMode) {
-        this.fastMode = fastMode;
+        this.sideEffectSet = fastMode ? SideEffectSet.none() : SideEffectSet.defaults();
     }
 
     /**