Added destructors to inputs

[obsgolem]

This is about the simplest way to solve this problem. Note that I do not do anything regarding making input structs non-Copy. Instead, you can handle free-after-free by using the Option return result.

I am not familiar enough with the codebase to know if this is likely to bork things elsewhere. I also don't know enough about our thread safety guarantees to be certain that the strategy of deleting each input ingredient in turn won't cause race conditions.

Resolves #345.

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[nikomatsakis]

Given all the recent churn, going to close, get in touch though if you want to update!