Runtime metadata construction

[FL33TW00D]

Some operations require their metadata struct to be constructed at runtime (we don't know the size of the metadata struct at compile time).

Concat is a good example, as there may be $0..n$ tensors being concatenated.
Therefore, we need some way of building the struct at runtime, whilst ensuring that we maintain the correct WGSL memory layout (play around here: https://webgpufundamentals.org/webgpu/lessons/resources/wgsl-offset-computer.html).

I'm currently thinking of the following:

use indexmap::IndexMap;
pub struct MetaMap(IndexMap<K, V, S = RandomState>)
where V: encase::ShaderSize

We can then convert this into a [u8] with members padded and offset correctly.

We would have needed to do this when we were JITting anyway.

[FL33TW00D]

Better solution: teoxoy/encase#46

Field rearranging would be interesting here, but it would save a few bytes, not worth it.

[FL33TW00D]

#137 solved, but isn't perfect. Thanks to @cwfitzgerald for the PR on encase.