process_wrapper: replace C++ implementation with rust.

[gigaroby]

The main advantages of this work are that it eliminates a bunch of
platform-specific C++ code in favor of the Rust standard library
(mainly std::process::Command and std::fs::File) and it simplifies
future improvements in the area of pipelining and incremental
compilation. Building a Rust process_wrapper requires a way to use
rust_binary without support from process_wrapper itself. This is
achieved through a transition in //util/process_wrapper/transitions.bzl.

[hlopko]

What a journey!!! Thank you!

[UebelAndre]

This change may have introduced a regression. d9ba374#commitcomment-68356750

@gigaroby @hlopko would either of you be able to take a look?

[ddeville]

fwiw I've been trying out the PR on Windows and I'm not having issues.

[dfreese]

Bisected to this commit, when trying to upgrade rules_rust. This triggered the following error for me:

    //proto/protobuf:protobuf_prost_generated (782077f4a782b271e6081e5d253d233307fbb065ebd24cbeda52275882be7e2c)
    //tools/prostgen:prostgen (ad7becf1f627c98f4c44ac49f5991ab41aa98662628ab4c26b029d7fb1035392)
.-> @rules_rust//util/process_wrapper:process_wrapper (ad7becf1f627c98f4c44ac49f5991ab41aa98662628ab4c26b029d7fb1035392)
|   @rules_rust//util/process_wrapper:process_wrapper_impl (ad7becf1f627c98f4c44ac49f5991ab41aa98662628ab4c26b029d7fb1035392)
|   @rules_rust//util/process_wrapper:process_wrapper_bin (ad7becf1f627c98f4c44ac49f5991ab41aa98662628ab4c26b029d7fb1035392)
`-- @rules_rust//util/process_wrapper:process_wrapper (ad7becf1f627c98f4c44ac49f5991ab41aa98662628ab4c26b029d7fb1035392)

Where the protobuf:protobuf_prost_generated target depends on the prostgen tool. This was in the host configuration. If I switched that to the exec configuration, the error went away, but it does seem like there's potentially a problem for rules that expected to be run on host.

[hlopko]

@dfreese - FYI from https://docs.bazel.build/versions/main/skylark/rules.html#configurations:

Historically, Bazel didn’t have the concept of execution platforms, and instead all build actions were considered to run on the host machine. Because of this, there is a single “host” configuration, and a “host” transition that can be used to build a dependency in the host configuration. Many rules still use the “host” transition for their tools, but this is currently deprecated and being migrated to use “exec” transitions where possible.

I think it's reasonable to proactively swift from using host to exec. Resiliency on transitions is one good reason (host doesn't work well with transitions - you can't transition from host).

[UebelAndre]

I think I found another related issue: #1209

(It's kinda cool though to see a panic 😄 Yay Rust!)