Use a custom allocator that allows casting of `Vec<T>` to `Vec<U>` (for different alignments)

[janpaul123]

This would be so useful in many places, especially when sending data to the GPU (where we typically need Vec<f32> and Vec<u32> buffers) and to JS, while still being able to use Rust structs to represent data in Rust.

We could make this part of a custom Rust stdlib (#66), especially to prevent users from overriding with a different allocator.