Draft: Allow configuring toolchain directory

Fixes #57.

Changes

• Renamed RustLinkPlugin to RustPlugin, as it now introduces the rust {} block.

• Renamed CargoHost to RustHost.

• Users can now configure the path of Rust installation with rust.installationDirectory.

  plugins {
    id("io.gitlab.trixnity.rust.kotlin.multiplatform")
  }
  
  rust {
    installationDirectory = File("/path/to/dir/containing/cargo/rustup")
  }

  This path is used when invoking cargo or rustup in the plugins.

To-Dos

• Redesign DSL

  kotlin {
    rust { useRustUpLinker() }
    rust { hostNativeTarget() }
  }