Fix interpreter segfault

[ChrisPenner]

Overview

We were getting some segfaults from the new unified stack builds, there was a small error in augSeg which I've fixed.

I also found a few other small things that are largely inconsequential but I threw in there:

• We previously hard-coded int-size to 8 bytes, now we use sizeOf @Int where appropriate
• Rewrote some imports to ensure we always use the array methods from Unison.Runtime.Array
• Fixed up array bounds checks to use non-deprecated size methods.

Test coverage

• [x] See if new build segfaults on cloud-client tests