Reduce stack usage of block splitter

[senhuang42]

This PR moves some of the block splitter stuff that was using too much stack space

I see no speed delta for levels 16-19 (have block splitter enabled by default) on enwik7 with gcc.

contrib/linux-kernel test
old: Maximum stack size: 2760
new: Maximum stack size: 1848

Todo: Fix fuzzer errors

[terrelln]

-Wcast-align has a point here. You'll need correct alignment here or certain platforms will fail.

This probably needs to go in the ZSTD_cwksp_reserve_object() space, or if that doesn't work just put it directly into the CCtx.

[senhuang42]

Yeah you're right. I think this will just rest directly in the CCtx. If I want to save space when we're not using block splitting, a wksp object doesn't seem to work properly with the different cctx reuse cases, since objects are assumed to remain the same throughout compressions.

If I wanted to make it work somehow, it would have to be the same as the other objects in that they stay the same throughout different cctx re-uses. And if we have to do that anyways, we may as well just keep it in the cctx. Though, this does mean we will just have to eat the >1K additional heap cost.

[terrelln]

Though, this does mean we will just have to eat the >1K additional heap cost.

That should be fine, thats much less than the total CCtx allocation. E.g. we have 500KB space for sequences, which is way overkill in most cases. So there is lower hanging fruit.

[senhuang42]

Rebased on top of #2788. For review, just consider the last commit in the stack.

[terrelln]

Looks good to me!

Before merging, you should rebase on top of dev, so you don't get any extra commits in the diff.