[perf] Do stack allocs instead of heap allocs for small fixed-size allocations

[MahdiBM]

For small fixed-size allocations, we can use stack instead of heap.

Motivation:

Allocations are costly. Stack allocations are much cheaper than heap allocations.

Modifications:

Use withUnsafeTemporaryAllocation(of:capacity:_:) instead of allocating an Array.
withUnsafeTemporaryAllocation allocates on the stack for up to 1000 bytes.

Result:

Better performance.

[Lukasa]

Nice improvement, thanks @MahdiBM.

[Lukasa]

The allocation counts went down (success!) so we need to update the integration test thresholds to lock the win in.