Use prebuilt Rust binaries

[lastmjs]

http://www.elebihan.com/posts/using-rust-with-buildroot-pre-built-binaries.html

For Kybra and Azle we'll eventually want to ship some kind of cargo/rustc installation script that can just pull down the prebuilt binaries. This would remove the need for the user to install Rust on their system.

[lastmjs]

We've cut subsequent compilation times in half, and we should be able to get Azle/Kybra down to ~5 seconds on subsequent compiles once the Wasm binary limit is increased. The problem that remains is the initial compile time, which is around 3-5 minutes now on my machine. I am going to bump the priority of this issue to medium for now.