Update dependencies

This MR updates and also removes some Rust dependencies for a release build:

• analysis_driver and gen_accessibility_driver binaries: Used for analysis's testing purposes only.
• derive_more crate: Keeps Display for future use.
• fs_extra crate: Unused and therefore removed.

The only interesting commits are !39 (75ba1933) and !39 (11f1254e).

It is now also possible to do cargo test (added in the CI), useful to add more tests.

Currently, cargo test fails on analysis: I'll look into this later (unless @lgaeher has already an idea why it might fail).

rr_frontend/analysis/tests/utils.rs

@@ -3,10 +3,18 @@ pub fn find_sysroot() -> String {
     // Taken from https://github.com/Manishearth/rust-clippy/pull/911.
     let home = option_env!("RUSTUP_HOME").or(option_env!("MULTIRUST_HOME"));
     let toolchain = option_env!("RUSTUP_TOOLCHAIN").or(option_env!("MULTIRUST_TOOLCHAIN"));
+
     match (home, toolchain) {
         (Some(home), Some(toolchain)) => format!("{}/toolchains/{}", home, toolchain),
         _ => option_env!("RUST_SYSROOT")
-            .expect("need to specify RUST_SYSROOT env var or use rustup or multirust")
+            .or_else(|| {
+                option_env!("RUST_SRC_PATH")
+                    .map(|s| s.strip_suffix("lib/rustlib/rustc-src/rust/compiler/rustc_driver/Cargo.toml"))
+                    .flatten()
+            })
+            .expect(
+                "Need to specify RUST_SYSROOT or RUST_SRC_PATH environment variables, or use rustup or multirust",
+            )
             .to_owned(),
     }
 }