Add fs.getCapacity and fs.attributes (#365)

- fs.getCapacity just returns the capacity of the current drive, if
   available. This will be nil on rom mounts.
 - fs.attributes returns an lfs like table of various file attributes.
   Currently, this contains:
    - access, modification, created: When this file was last accessed,
      modified and created. Time is measured in milliseconds since the
      epoch, same as os.epoch("utc") and what is accepted by os.date.
    - size: Same as fs.getSize
    - isDir: same as fs.isDir

Closes #262

src/main/java/dan200/computercraft/api/filesystem/FileAttributes.java

@@ -0,0 +1,81 @@
+/*
+ * This file is part of the public ComputerCraft API - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2020. This API may be redistributed unmodified and in full only.
+ * For help using the API, and posting your mods, visit the forums at computercraft.info.
+ */
+package dan200.computercraft.api.filesystem;
+
+import java.nio.file.attribute.BasicFileAttributes;
+import java.nio.file.attribute.FileTime;
+import java.time.Instant;
+
+/**
+ * A simple version of {@link BasicFileAttributes}, which provides what information a {@link IMount} already exposes.
+ */
+final class FileAttributes implements BasicFileAttributes
+{
+    private static final FileTime EPOCH = FileTime.from( Instant.EPOCH );
+
+    private final boolean isDirectory;
+    private final long size;
+
+    FileAttributes( boolean isDirectory, long size )
+    {
+        this.isDirectory = isDirectory;
+        this.size = size;
+    }
+
+    @Override
+    public FileTime lastModifiedTime()
+    {
+        return EPOCH;
+    }
+
+    @Override
+    public FileTime lastAccessTime()
+    {
+        return EPOCH;
+    }
+
+    @Override
+    public FileTime creationTime()
+    {
+        return EPOCH;
+    }
+
+    @Override
+    public boolean isRegularFile()
+    {
+        return !isDirectory;
+    }
+
+    @Override
+    public boolean isDirectory()
+    {
+        return isDirectory;
+    }
+
+    @Override
+    public boolean isSymbolicLink()
+    {
+        return false;
+    }
+
+    @Override
+    public boolean isOther()
+    {
+        return false;
+    }
+
+    @Override
+    public long size()
+    {
+        return size;
+    }
+
+    @Override
+    public Object fileKey()
+    {
+        return null;
+    }
+}

src/main/java/dan200/computercraft/api/filesystem/IMount.java

@@ -14,6 +14,7 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.nio.channels.Channels;
 import java.nio.channels.ReadableByteChannel;
+import java.nio.file.attribute.BasicFileAttributes;
 import java.util.List;
 
 /**
@@ -93,4 +94,18 @@ public interface IMount
     {
         return Channels.newChannel( openForRead( path ) );
     }
+
+    /**
+     * Get attributes about the given file.
+     *
+     * @param path The path to query.
+     * @return File attributes for the given file.
+     * @throws IOException If the file does not exist, or attributes could not be fetched.
+     */
+    @Nonnull
+    default BasicFileAttributes getAttributes( @Nonnull String path ) throws IOException
+    {
+        if( !exists( path ) ) throw new FileOperationException( path, "No such file" );
+        return new FileAttributes( isDirectory( path ), getSize( path ) );
+    }
 }

src/main/java/dan200/computercraft/api/filesystem/IWritableMount.java

@@ -14,6 +14,7 @@ import java.io.IOException;
 import java.io.OutputStream;
 import java.nio.channels.Channels;
 import java.nio.channels.WritableByteChannel;
+import java.util.OptionalLong;
 
 /**
  * Represents a part of a virtual filesystem that can be mounted onto a computer using {@link IComputerAccess#mount(String, IMount)}
@@ -105,4 +106,16 @@ public interface IWritableMount extends IMount
      * @throws IOException If the remaining space could not be computed.
      */
     long getRemainingSpace() throws IOException;
+
+    /**
+     * Get the capacity of this mount. This should be equal to the size of all files/directories on this mount, minus
+     * the {@link #getRemainingSpace()}.
+     *
+     * @return The capacity of this mount, in bytes.
+     */
+    @Nonnull
+    default OptionalLong getCapacity()
+    {
+        return OptionalLong.empty();
+    }
 }