significantly speed up application snapshots plugin

[bconstanzo]

Solved the optimization of the ASTC decompression library that I had in mind before. I made a fork of astc_decomp, optimized, and published it. This PR changes the astc_decomp import in the ios_ktx2png script to astc_decomp_faster.

There's also a small change to take (and show) wall time when running ileapp. That helped me make sense of a few optimizations and benchmarks because sometimes processing time doesn't show the full picture.