Use pure-rust protoc

[eira-fransham]

Closes #183

This was basically a couple hours of grep + sed pipelines and running cargo test until it compiled

[tomaka]

Shouldn't we generate the Rust files in the directory pointed by the OUT env var (idiomatic cargo)?
https://doc.rust-lang.org/cargo/reference/build-scripts.html#case-study-code-generation

[jamesray1]

Unfortunately (like #259), this is also failing on Windows for me.

I can confirm that the diff locally between this PR and master is the same. Here is the cargo build output.

[eira-fransham]

@jamesray1

process didn't exit successfully: `C:\Users\James\jrl\target\debug\build\ring-4327ef7c900cd242\build-script-build` (exit code: 101)

This is ring failing and has nothing to do with protoc.

@tomaka

I mentioned this on Riot but for posterity: rust-lang/rfcs#752 blocks us from doing this properly. There's a plugin implementing a workaround but A) it requires nightly and B) it requires an old nightly (it doesn't work anymore)

[tomaka]

I suggest to not close #183 (or reopen it after merging) so that we fix that correctly eventually.