Consider a container for arrays `[T; N]`

[antiguru]

At the moment, we can store arrays ([T; N]) in slice containers. Slice containers absorb variably-sized elements, and to determine an element's size, it keeps a separate storage for index bounds around.

For fixed-length arrays, we could instead use a specialized container that doesn't maintain bounds, since each element would have the same constant length.

[frankmcsherry]

I did this over in datatoad, and here is my report:

1. This was very, very helpful for performance. Bounds checks especially were in the way of performant sorting and searching. The performant version used a custom container, roughly Arrays<const K: usize>, and wasn't as modular as the next attempt ...
2. I did this another way with a custom bounds container in a Vecs, which is now possible. The bounds container was roughly Array<const K: usize>(usize), where the integer is the length of the values list, which the bounds container needs to hold (though we know could be determined from the length of the values container). I didn't get around to trying this, yet, but it should be just a little bit slower than the custom version, with some caveats around bounds tests on lengths (the first version used the length of the values container, vs a random integer it has).

Not sure what to conclude at the moment, but the good news is that it seemed not too hard to try either or both of these. I agree that the current container, a Vecs<VC> with a Vec<usize> for the bounds, is not nearly as good as what would probably be a very short BC to type up.