Add actual use of the `struct Structure` lying dormant in `new-box-syntax.rs`.

The original test program exercised the (garbage-collected heap)
allocation form `box (GC) ...`, feeding an instance of `Structure` in
as the expression. This obviously went away when `GC` went away, but
there's no reason for us not to include an appropriate unit test here
for the same form, just for heap-allocated instances of `Structure`.

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