perf: optimize error type

[Noah-Kennedy]

The allocations and frees for these errors can get expensive in some cases. Any reduction in allocator usage here helps some use cases a lot it seems.

[Noah-Kennedy]

I'm leaving this as a draft while I look into ways to maybe make Cause faster as well.

[Noah-Kennedy]

H2 is probably the most likely to cause perf issues for various reasons, so I'm focusing on that case for now.

[Noah-Kennedy]

Marked as ready and squashing to make review easier. Gonna leave as two commits and squash again on merge.

[tottoto]

Looks good to me. However, as it increases a little complexity, I would think that there would needs to be a benchmark to show in which cases it is actually efficient or not, and how the efficiency is.

[seanmonstar]

For context, the reason it was boxed before was:

• To reduce return sizes of Result<T, Error>.
• Because errors should be less common, it seemed better to make the error case more expensive if it allowed the good case to be any faster.

[Noah-Kennedy]

Looks good to me. However, as it increases a little complexity, I would think that there would needs to be a benchmark to show in which cases it is actually efficient or not, and how the efficiency is.

I'll try and construct something, although I suspect that for most users this won't matter. That said, the effect of the double allocation is surprisingly significant.

[Noah-Kennedy]

For context, the reason it was boxed before was:

* To reduce return sizes of `Result<T, Error>`.

* Because errors should be less common, it seemed better to make the error case more expensive if it allowed the good case to be any faster.

Hmm, the non-error case is a good point. I'll see if I can shrink this down.

I'm investigating more right now the actual production issue I saw with this, because there's a chance that this might not be a valid use of HTTP/2 semantics I'm looking at which is causing this. I might potentially close this if I decide that any case where this matters is an issue of "you're holding it wrong".

[seanmonstar]

Did it turn out this isn't desired?