make D[x,y...] correspond to D[(x,y...)] for a dictionary D

As discussed in #4803, this makes
```
mydict[key1, key2, key3]
```
equivalent to
```
mydict[(key1, key2, key3)]
```
to give a convenient syntax for multi-index dictionaries analogous to multi-index arrays.  

See the discussion in #4803.  This is apparently the Python behavior as well.