Once again, the interaction between anchors and extras (specifically
comments) was causing trouble.
The root of the problem was the fact that in `a[b]`, we put `b` in the
`index` field of the subscript node, whereas in `a[b,c]`, we
additionally synthesize a `Tuple` node for `b,c` (which matches the
Python AST).
To fix this, we refactored the grammar slightly so as to make that tuple
explicit, such that a subscript node either contains a single expression
or the newly added tuple node. This greatly simplifies the logic.
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