Move API registration into separate classes

There's several reasons for this change:
 - Try to make ComputerCraft.java less monolithic by moving
   functionality into separate module-specific classes.
 - Hopefully make the core class less Minecraft dependent, meaning
   emulators are a little less dependent on anything outside of /core.

Note we still need /some/ methods in the main ComputerCraft class in
order to maintain backwards compatibility with Plethora and
Computronics.

src/main/java/dan200/computercraft/shared/PocketUpgrades.java

@@ -0,0 +1,71 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2018. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+
+package dan200.computercraft.shared;
+
+import com.google.common.base.Preconditions;
+import dan200.computercraft.ComputerCraft;
+import dan200.computercraft.api.pocket.IPocketUpgrade;
+import dan200.computercraft.api.turtle.ITurtleUpgrade;
+import dan200.computercraft.shared.computer.core.ComputerFamily;
+import dan200.computercraft.shared.util.InventoryUtil;
+import net.minecraft.item.ItemStack;
+
+import javax.annotation.Nonnull;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+public final class PocketUpgrades
+{
+    private static final Map<String, IPocketUpgrade> upgrades = new HashMap<>();
+
+    public static void register( @Nonnull IPocketUpgrade upgrade )
+    {
+        Preconditions.checkNotNull( upgrade, "upgrade cannot be null" );
+
+        String id = upgrade.getUpgradeID().toString();
+        IPocketUpgrade existing = upgrades.get( id );
+        if( existing != null )
+        {
+            throw new IllegalStateException( "Error registering '" + upgrade.getUnlocalisedAdjective() + " pocket computer'. UpgradeID '" + id + "' is already registered by '" + existing.getUnlocalisedAdjective() + " pocket computer'" );
+        }
+
+        upgrades.put( id, upgrade );
+    }
+
+
+    public static IPocketUpgrade get( String id )
+    {
+        return upgrades.get( id );
+    }
+
+    public static IPocketUpgrade get( @Nonnull ItemStack stack )
+    {
+        if( stack.isEmpty() ) return null;
+
+        for( IPocketUpgrade upgrade : upgrades.values() )
+        {
+            ItemStack craftingStack = upgrade.getCraftingItem();
+            if( !craftingStack.isEmpty() && InventoryUtil.areItemsStackable( stack, craftingStack ) )
+            {
+                return upgrade;
+            }
+        }
+
+        return null;
+    }
+
+    public static Iterable<IPocketUpgrade> getVanillaUpgrades()
+    {
+        List<IPocketUpgrade> vanilla = new ArrayList<>();
+        vanilla.add( ComputerCraft.PocketUpgrades.wirelessModem );
+        vanilla.add( ComputerCraft.PocketUpgrades.advancedModem );
+        vanilla.add( ComputerCraft.PocketUpgrades.pocketSpeaker );
+        return vanilla;
+    }
+}