Expose Computer.getRootMount again

It's a little evil, but we need it for CCEmuX. There's other ways of
achieving this, but not with supporting CC and CC:T.

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

@@ -7,6 +7,7 @@
 package dan200.computercraft.core.computer;
 
 import com.google.common.base.Objects;
+import dan200.computercraft.api.filesystem.IWritableMount;
 import dan200.computercraft.api.lua.ILuaAPI;
 import dan200.computercraft.api.peripheral.IPeripheral;
 import dan200.computercraft.api.peripheral.IWorkMonitor;
@@ -244,6 +245,12 @@ public void advance( double dt )
         tick();
     }
 
+    @Deprecated
+    public IWritableMount getRootMount()
+    {
+        return executor.getRootMount();
+    }
+
     @Deprecated
     public static final String[] s_sideNames = IAPIEnvironment.SIDE_NAMES;
 }

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

@@ -338,7 +338,7 @@ private IMount getRomMount()
         }
     }
 
-    private FileSystem createFileSystem()
+    IWritableMount getRootMount()
     {
         if( rootMount == null )
         {
@@ -347,11 +347,15 @@ private FileSystem createFileSystem()
                 computer.getComputerEnvironment().getComputerSpaceLimit()
             );
         }
+        return rootMount;
+    }
 
+    private FileSystem createFileSystem()
+    {
         FileSystem filesystem = null;
         try
         {
-            filesystem = new FileSystem( "hdd", rootMount );
+            filesystem = new FileSystem( "hdd", getRootMount() );
 
             IMount romMount = getRomMount();
             if( romMount == null )