Auto merge of #118490 - Nadrieril:arena-alloc-matrix, r=nnethercote

Exhaustiveness: allocate memory better

Exhaustiveness is a recursive algorithm that allocates a bunch of slices at every step. Let's see if I can improve performance by improving allocations.

Already just using `Vec::with_capacity` is showing impressive improvements on my local measurements.

r? `@ghost`

Author: bors