Re-add original SD FAT info access methods (#6092)

Fixes #6081

The SD rewrite blanked out some of the internal FAT info.. Restore the
function calls and return proper values.

Because size() is used in many printf()s, we can't just change its
return type to uint64.  Instead, when size is > size-max warn.

Add SD.size64 which can be used by new apps who care about >4GB cards.
Prints a warning if debugging enabled

cores/esp8266/FS.h

@@ -24,6 +24,8 @@
 #include <memory>
 #include <Arduino.h>
 
+class SDClass;
+
 namespace fs {
 
 class File;
@@ -208,8 +210,10 @@ public:
 
     bool gc();
 
+    friend class ::SDClass; // More of a frenemy, but SD needs internal implementation to get private FAT bits
 protected:
     FSImplPtr _impl;
+    FSImplPtr getImpl() { return _impl; }
 };
 
 } // namespace fs