[zapps-wg] Trusted Rust build process is ready for evaluation

[Devrandom]

Hi all,

I was able to build rustc completely from sources.  This means no network
access during the build and without any Rust distributed binaries.

The steps are here:

https://github.com/devrandom/powersoftau/wiki/Trusted-build-instructions-via-mrustc

I'm still looking into building a cargo binary that supports TLS, to
eliminate the need to download it for building powersoftau itself.  Worst
case we can use a shell script.

Would like to merge https://github.com/ebfull/pairing/pull/72 so that we
don't have to maintain a separate patch for Rust 1.19 compatibility.

Sean, what is the best method of validating that the generated binary works
correctly?  Just use "new", then "compute" and then "verify_transform"?

Also, I found that the generated binary is *very* different from the one
generated by the downloadable Rust compiler.  This is somewhat worrisome,
although it doesn't affect us if we use the mrustc path.  Will spend a bit
more time to get to the bottom of it.