Replace validation visitor stacks with single-slot variables

[swalkinshaw]

Extracted #5578

Eliminates array push/pop operations per validation pass by replacing array-based stacks with single-slot variables that use the Ruby call stack for save/restore. Pre-allocates path array indexed by depth counter.

In our schema, this saves about ~5500 push/pop operations and resulted in a 5-6% perf improvement.

I'll defer to you if you think this makes the code more complex and harder to manage but I think it's about the same.

[rmosolgo]

This is interesting: IIUC, instead of dynamically growing and shrinking the Array, we're using a big array with the expectation that only the first @path_depth items are correct. Instead of appending a new item to @path, call sites assign a value in @path at the current @path_depth then increment @path_depth.

Just curious, is there any reason for 32 in particular?

[swalkinshaw]

I think in isolation this had a tiny improvement but in the context of removing the push/pop operations already I think going back to [] is just simpler. Ruby doubles the array size as it grows so really this might avoid like 3-4 resizes (a realloc + memcpy of a few pointers?) which is nothing. Plus for small queries it's over allocating technically.

Removed!

[rmosolgo]

I definitely didn't mean to argue against starting at 32 slots, I was just genuinely curious why 32. I thought I might learn some Ruby secrets! Either way is fine for me.