[WIP] Apply &on-heap selectively
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rsmmr
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The parser generator is creating temporaries at a few places that will then later receive parsed values. Currently, those temporaries are always simply of the type being parsed. Turns out that works for units/structs only because we're always wrapping them into `value_ref`, meaning we can create those temporaries as null values. However, in a subsequent commit, we will get rid of those wrappings in some cases, meaning the structs would need to be default initialized at the time they are created on the stack. That doesn't work for some struct types though: if a struct receives parameters, we would need to pass them at instantiation time, but for these temporaries we don't know parameters at that point. So to prepare for the upcoming change, this commit wraps such temporary struct values into `optional<>` iff they receive parameters. (I have been wondering if we should generally wrap all these parser temporaries into optionals, independent of their type. Semantically that would make sense, but I'm guessing the C++ compiler gets better optimization opportunties if we don't.)
Previously, we would attach `&on-heap` to any `struct` compiled from a Spicy `unit`. Now we only attach it only for specific cases that need it, moving state to the stack otherwise.
For efficiency and consistency.
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In our generated C++ parsing code, so far we generally moved all units
to the heap. This PR changes that to do so only for cases where we
actually need it (primarily: if we wrap the unit into a reference
somewhere, or if the unit type is cyclic). The assumption is that this
should improve both performance and memory usage.
&on-heapselectively only where needed.setto anunordered_set.