Drop 1.16.5 support to use Mojang mappings

Because there is no option to create a `remapped-mojang` artifact for 1.16.5 and the whole point of the scripts is to save me time, 1.16.5 support is being removed earlier than usual.

Also fixes issues with NMS-based shulker collision check.
Closes #36

api/src/main/java/com/lishid/openinv/util/InventoryAccess.java

@@ -20,7 +20,6 @@ import com.lishid.openinv.internal.IInventoryAccess;
 import com.lishid.openinv.internal.ISpecialEnderChest;
 import com.lishid.openinv.internal.ISpecialInventory;
 import com.lishid.openinv.internal.ISpecialPlayerInventory;
-import java.lang.reflect.Field;
 import java.lang.reflect.Method;
 import org.bukkit.Bukkit;
 import org.bukkit.inventory.Inventory;
@@ -81,7 +80,8 @@ public class InventoryAccess implements IInventoryAccess {
             } catch (ReflectiveOperationException ignored) {}
         }
 
-        inv = grabFieldOfTypeFromObject(expected, inventory);
+        // Use reflection to find the IInventory
+        inv = ReflectionHelper.grabObjectByType(inventory, expected);
 
         if (expected.isInstance(inv)) {
             return expected.cast(inv);
@@ -90,23 +90,6 @@ public class InventoryAccess implements IInventoryAccess {
         return null;
     }
 
-    private static <T> @Nullable T grabFieldOfTypeFromObject(final Class<T> type, final Object object) {
-        // Use reflection to find the IInventory
-        Class<?> clazz = object.getClass();
-        T result = null;
-        for (Field f : clazz.getDeclaredFields()) {
-            f.setAccessible(true);
-            if (type.isAssignableFrom(f.getDeclaringClass())) {
-                try {
-                    result = type.cast(f.get(object));
-                } catch (Exception e) {
-                    e.printStackTrace();
-                }
-            }
-        }
-        return result;
-    }
-
     @Deprecated
     @Override
     public @Nullable ISpecialEnderChest getSpecialEnderChest(@NotNull Inventory inventory) {

api/src/main/java/com/lishid/openinv/util/ReflectionHelper.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2011-2021 lishid. All rights reserved.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 3.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.lishid.openinv.util;
+
+import java.lang.reflect.Field;
+import org.jetbrains.annotations.Nullable;
+
+/**
+ * A utility for making reflection easier.
+ */
+public final class ReflectionHelper {
+
+    private ReflectionHelper() {}
+
+    /**
+     * Grab an {@link Object} stored in a {@link Field} of another {@code Object}.
+     *
+     * <p>This casts the field to the correct class. Any issues will result in a {@code null} return value.
+     *
+     * @param fieldType the {@link Class} of {@code Object} stored in the {@code Field}
+     * @param holder the containing {@code Object}
+     * @param <T> the type of stored {@code Object}
+     * @return the first matching {@code Object} or {@code null} if none match
+     */
+    public static <T> @Nullable T grabObjectByType(final Object holder, final Class<T> fieldType) {
+        Field field = grabFieldByType(holder.getClass(), fieldType);
+
+        if (field != null) {
+            try {
+                return fieldType.cast(field.get(holder));
+            } catch (IllegalAccessException ignored) {
+                // Ignore issues obtaining field
+            }
+        }
+
+        return null;
+    }
+
+    /**
+     * Grab a {@link Field} of an {@link Object}
+     *
+     * @param fieldType the {@link Class} of the object
+     * @param holderType the containing {@code Class}
+     * @return the first matching object or {@code null} if none match
+     */
+    public static @Nullable Field grabFieldByType(Class<?> holderType, Class<?> fieldType) {
+        for (Field field : holderType.getDeclaredFields()) {
+            field.setAccessible(true);
+            if (fieldType.isAssignableFrom(field.getType())) {
+                return field;
+            }
+        }
+
+        if (holderType.getSuperclass() != null) {
+            return grabFieldByType(fieldType, holderType.getSuperclass());
+        }
+
+        return null;
+    }
+
+}