Refactor the filesystem and HTTP code

- Move the encoding/decoding from the Filesystem implementation to the
   individual handles.
 - Move each handle into an core.apis.handles package from the main fs
   API.
 - Move the HTTP response to inherit from these handles.
 - Allow binary handles' read function to accept a number, specifying
   how many characters to read - these will be returned as a Lua string.
 - Add readAll to binary handles
 - Allow binary handles' write function to accept a string which is
   decoded into the individual bytes.
 - Add "binary" argument to http.request and friends in order to return
   a binary handle.
 - Ensure file handles are open when reading from/writing to them.
 - Return the error message when opening a file fails.

src/main/java/dan200/computercraft/shared/util/StringUtil.java

@@ -41,4 +41,17 @@ public class StringUtil
     {
         return net.minecraft.util.text.translation.I18n.translateToLocalFormatted( key, format );
     }
+
+    public static byte[] encodeString( String string )
+    {
+        byte[] chars = new byte[ string.length() ];
+
+        for( int i = 0; i < chars.length; ++i )
+        {
+            char c = string.charAt( i );
+            chars[ i ] = c < 256 ? (byte) c : 63;
+        }
+
+        return chars;
+    }
 }