Provide a more direct way to get the related computer

Effectively shift extracting the computer away from Plethora into CC:T.
Ideally we wouldn't need this at all, but Plethora does some funky
things with tick timings.

See SquidDev-CC/plethora#125

src/main/java/dan200/computercraft/core/apis/ComputerAccess.java

@@ -3,14 +3,17 @@ package dan200.computercraft.core.apis;
 import dan200.computercraft.api.filesystem.IMount;
 import dan200.computercraft.api.filesystem.IWritableMount;
 import dan200.computercraft.api.peripheral.IComputerAccess;
+import dan200.computercraft.core.computer.Computer;
+import dan200.computercraft.core.computer.IComputerOwned;
 import dan200.computercraft.core.filesystem.FileSystem;
 import dan200.computercraft.core.filesystem.FileSystemException;
 
 import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 import java.util.HashSet;
 import java.util.Set;
 
-public abstract class ComputerAccess implements IComputerAccess
+public abstract class ComputerAccess implements IComputerAccess, IComputerOwned
 {
     private final IAPIEnvironment m_environment;
     private final Set<String> m_mounts = new HashSet<>();
@@ -133,6 +136,13 @@ public abstract class ComputerAccess implements IComputerAccess
         m_environment.queueEvent( event, arguments );
     }
 
+    @Nullable
+    @Override
+    public Computer getComputer()
+    {
+        return m_environment.getComputer();
+    }
+
     private String findFreeLocation( String desiredLoc )
     {
         try

src/main/java/dan200/computercraft/core/apis/IAPIEnvironment.java

@@ -9,17 +9,19 @@ package dan200.computercraft.core.apis;
 import dan200.computercraft.api.peripheral.IPeripheral;
 import dan200.computercraft.core.computer.Computer;
 import dan200.computercraft.core.computer.IComputerEnvironment;
+import dan200.computercraft.core.computer.IComputerOwned;
 import dan200.computercraft.core.filesystem.FileSystem;
 import dan200.computercraft.core.terminal.Terminal;
 import dan200.computercraft.core.tracking.TrackingField;
 
-public interface IAPIEnvironment
+public interface IAPIEnvironment extends IComputerOwned
 {
     interface IPeripheralChangeListener
     {
         void onPeripheralChanged( int side, IPeripheral newPeripheral );
     }
-    
+
+    @Override
     Computer getComputer();
     int getComputerID();
     IComputerEnvironment getComputerEnvironment();

src/main/java/dan200/computercraft/core/computer/IComputerOwned.java

@@ -0,0 +1,9 @@
+package dan200.computercraft.core.computer;
+
+import javax.annotation.Nullable;
+
+public interface IComputerOwned
+{
+    @Nullable
+    Computer getComputer();
+}

src/main/java/dan200/computercraft/shared/peripheral/modem/WiredModemPeripheral.java

@@ -10,6 +10,8 @@ import dan200.computercraft.api.network.wired.IWiredNode;
 import dan200.computercraft.api.network.wired.IWiredSender;
 import dan200.computercraft.api.peripheral.IComputerAccess;
 import dan200.computercraft.api.peripheral.IPeripheral;
+import dan200.computercraft.core.computer.Computer;
+import dan200.computercraft.core.computer.IComputerOwned;
 import net.minecraft.world.World;
 
 import javax.annotation.Nonnull;
@@ -273,7 +275,7 @@ public abstract class WiredModemPeripheral extends ModemPeripheral implements IW
         throw new LuaException( "No peripheral: " + remoteName );
     }
 
-    private static class RemotePeripheralWrapper implements IComputerAccess
+    private static class RemotePeripheralWrapper implements IComputerAccess, IComputerOwned
     {
         private final WiredModemElement m_element;
         private final IPeripheral m_peripheral;
@@ -408,5 +410,12 @@ public abstract class WiredModemPeripheral extends ModemPeripheral implements IW
                 return m_element.getRemotePeripherals().get( name );
             }
         }
+
+        @Nullable
+        @Override
+        public Computer getComputer()
+        {
+            return m_computer instanceof IComputerOwned ? ((IComputerOwned) m_computer).getComputer() : null;
+        }
     }
 }