Performance: Apply post-link optimization (PLO)

[reneleonhardt]

Problem

PLO seems to be working really well to improve performance, especially for larger binaries.

Proposed Solution

With Propeller there already is a tool being used in production by many projects.

Notes

Propeller may eventually be upstreamed, making it even easier to use in cargo.
https://www.phoronix.com/news/Google-Wants-Up-LLVM-Propeller