In situ parsing WIP

[pickfire]

dtolnay/request-for-implementation#7

I believe it would be better to make this function unsafe since it would be undefined behavior if they tried to call this function twice on the same data unless we can somehow make sure they do not call it twice. Pending tasks:

• remove unused benchmark
• remove fuzz tests (not sure if useful later)
• make function unsafe (needs discussion)
• update docs
• rearrange tests

[dtolnay]

This looks like a promising start.

I'll close the PR because I'm not interested in accepting this functionality in serde_json just yet. I would prefer if this were published as a separate crate -- that's why it was filed in request-for-implementation instead of an issue here.

Thanks anyway!