Rust updates

[str4d]

No description provided.

[gmale]

I started this review optimistic about the Mozilla plugin but ended the review far less hopeful. Now, I'm beginning to like your original idea of creating gradle tasks ourselves.

It would take less than a half-day to convert our shell/batch scripts to a gradle plugin and it might not be a bad idea for Zcash to maintain an android-rust gradle plugin, similar to mozilla's but modernized with more functionality for non-rust developers (i.e. most Android teams) AND leveraging the new features in R19 which deprecated the need for make_standalone_toolchains.py

[gmale]

there are two ways to declare plugins, we probably don't want to mix approaches. So we may want to change this to:

apply plugin: 'org.mozilla.rust-android-gradle.rust-android'

and then add the plugin to the buildscript dependencies like so per the readme usage section.:

buildscript {
    dependencies {
        classpath 'gradle.plugin.org.mozilla.rust-android-gradle:plugin:0.8.0'
    }
}

HOWEVER, I'd rather switch all the other plugins to using this style that you have here, instead, because it's cleaner and automatically adds the the buildscript dependency. I was using the other style because one of the plugins required it but I think I've since dropped that plugin in the latest version of this build file.

[str4d]

I agree we should use the modern approach rather than legacy.

[str4d]

it might not be a bad idea for Zcash to maintain an android-rust gradle plugin, similar to mozilla's but modernized with more functionality for non-rust developers (i.e. most Android teams) AND leveraging the new features in R19 which deprecated the need for make_standalone_toolchains.py

I think it would be valuable to fork the Mozilla plugin and modify it as necessary instead of making our own plugin, because then we can offer the changes back upstream. If they accept them, then the wider ecosystem benefits, and we don't have to maintain a fork. If they don't accept them, then we're in the same position we would have been anyway.

[str4d]

Rebased on master and removed the build system change, because #16 would conflict with it, and I'm still figuring out how to use a Gradle plugin from a GitHub branch.

[gmale]

LGTM