Some API cleanup

- Remove *Stream methods on IMount/IWritableMount, and make the channel
   ones the primary.
 - Fix location of AbstractTurtleUpgrade
 - Make IComputerAccess.getAvailablePeripheral and .getMainThreadMonitor
   mandatory.
 - IComputerAccess throws a specialised NotAttachedException

src/test/java/dan200/computercraft/core/ComputerTestDelegate.java

@@ -86,7 +86,7 @@ public class ComputerTestDelegate
         for( String child : children ) mount.delete( child );
 
         // And add our startup file
-        try( WritableByteChannel channel = mount.openChannelForWrite( "startup.lua" );
+        try( WritableByteChannel channel = mount.openForWrite( "startup.lua" );
              Writer writer = Channels.newWriter( channel, StandardCharsets.UTF_8.newEncoder(), -1 ) )
         {
             writer.write( "loadfile('test/mcfly.lua', nil, _ENV)('test/spec') cct_test.finish()" );

src/test/java/dan200/computercraft/core/filesystem/JarMountTest.java

@@ -13,14 +13,14 @@ import org.junit.jupiter.api.Test;
 import java.io.File;
 import java.io.FileOutputStream;
 import java.io.IOException;
-import java.io.InputStream;
+import java.nio.channels.Channels;
+import java.nio.channels.ReadableByteChannel;
 import java.nio.charset.StandardCharsets;
 import java.util.zip.ZipEntry;
 import java.util.zip.ZipOutputStream;
 
 import static org.junit.jupiter.api.Assertions.*;
 
-@SuppressWarnings( "deprecation" )
 public class JarMountTest
 {
     private static final File ZIP_FILE = new File( "test-files/jar-mount.zip" );
@@ -63,9 +63,9 @@ public class JarMountTest
     {
         IMount mount = new JarMount( ZIP_FILE, "dir/file.lua" );
         byte[] contents;
-        try( InputStream stream = mount.openForRead( "" ) )
+        try( ReadableByteChannel stream = mount.openForRead( "" ) )
         {
-            contents = ByteStreams.toByteArray( stream );
+            contents = ByteStreams.toByteArray( Channels.newInputStream( stream ) );
         }
 
         assertEquals( new String( contents, StandardCharsets.UTF_8 ), "print('testing')" );
@@ -76,9 +76,9 @@ public class JarMountTest
     {
         IMount mount = new JarMount( ZIP_FILE, "dir" );
         byte[] contents;
-        try( InputStream stream = mount.openForRead( "file.lua" ) )
+        try( ReadableByteChannel stream = mount.openForRead( "file.lua" ) )
         {
-            contents = ByteStreams.toByteArray( stream );
+            contents = ByteStreams.toByteArray( Channels.newInputStream( stream ) );
         }
 
         assertEquals( new String( contents, StandardCharsets.UTF_8 ), "print('testing')" );

src/test/java/dan200/computercraft/core/filesystem/MemoryMount.java

@@ -6,9 +6,15 @@
 package dan200.computercraft.core.filesystem;
 
 import dan200.computercraft.api.filesystem.IWritableMount;
+import dan200.computercraft.core.apis.handles.ArrayByteChannel;
 
 import javax.annotation.Nonnull;
-import java.io.*;
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.IOException;
+import java.nio.channels.Channels;
+import java.nio.channels.ReadableByteChannel;
+import java.nio.channels.WritableByteChannel;
 import java.util.*;
 
 /**
@@ -61,10 +67,9 @@ public class MemoryMount implements IWritableMount
 
     @Nonnull
     @Override
-    @Deprecated
-    public OutputStream openForWrite( @Nonnull final String path )
+    public WritableByteChannel openForWrite( @Nonnull final String path )
     {
-        return new ByteArrayOutputStream()
+        return Channels.newChannel( new ByteArrayOutputStream()
         {
             @Override
             public void close() throws IOException
@@ -72,13 +77,12 @@ public class MemoryMount implements IWritableMount
                 super.close();
                 files.put( path, toByteArray() );
             }
-        };
+        } );
     }
 
     @Nonnull
     @Override
-    @Deprecated
-    public OutputStream openForAppend( @Nonnull final String path ) throws IOException
+    public WritableByteChannel openForAppend( @Nonnull final String path ) throws IOException
     {
         ByteArrayOutputStream stream = new ByteArrayOutputStream()
         {
@@ -93,7 +97,7 @@ public class MemoryMount implements IWritableMount
         byte[] current = files.get( path );
         if( current != null ) stream.write( current );
 
-        return stream;
+        return Channels.newChannel( stream );
     }
 
     @Override
@@ -131,10 +135,9 @@ public class MemoryMount implements IWritableMount
 
     @Nonnull
     @Override
-    @Deprecated
-    public InputStream openForRead( @Nonnull String path )
+    public ReadableByteChannel openForRead( @Nonnull String path )
     {
-        return new ByteArrayInputStream( files.get( path ) );
+        return new ArrayByteChannel( files.get( path ) );
     }
 
     public MemoryMount addFile( String file, String contents )