The exression type cannot be a member of a struct that is an
element of the expression variant. This is because it would then
be required to contain itself. So introduce a nested_expression
type to indirectly hold an expression, but keep the value semantics
we expect from expressions: it is copyable and a copy has separate
identity and storage.
In fact binary_operator had to resort to this trick, so it's converted
to nested_expression in the next patch.
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