scripts: Removed item/iter magic methods from fs object classes

So no more __getitem__, __contains__, or __iter__ for Rbyd, Btree, Mdir,
Mtree, Lfs.File, etc.

These were way too error-prone, especially when accidental unpacking
triggered unintended disk traversal and weird error states. We didn't
even use the implicit behavior because we preferred the full name for
heavy disk operations.

The motivation for this was Python not catching this bug, which is a bit
silly:

  rid, rattr, *path_ = rbyd

scripts/dbgrbyd.py

@@ -791,18 +791,6 @@ class Rbyd:
         else:
             return rattr_
 
-    def __getitem__(self, key):
-        if not isinstance(key, tuple):
-            key = (key,)
-
-        return self.lookup(*key)
-
-    def __contains__(self, key):
-        if not isinstance(key, tuple):
-            key = (key,)
-
-        return self.lookup(*key) is not None
-
     def rids(self, *,
             path=False):
         rid = -1
@@ -869,9 +857,6 @@ class Rbyd:
                     yield rattr_
                 tag_ = rattr_.tag
 
-    def __iter__(self):
-        return self.rattrs()
-
     # lookup by name
     def namelookup(self, did, name):
         # binary search
@@ -1647,7 +1632,7 @@ def dbg_tree(rbyd, *,
     # dynamically size the id field
     w_width = mt.ceil(mt.log10(max(1, rbyd.weight)+1))
 
-    for i, (rid, rattr) in enumerate(rbyd):
+    for i, (rid, rattr) in enumerate(rbyd.rattrs()):
         # show human-readable tag representation
         print('%08x: %s%*s %-*s  %s' % (
                 rattr.toff,