Initial reorg.

This is largely the same as #305, but updated for the current tree.

crates/environ/build.rs

@@ -0,0 +1,9 @@
+use std::process::Command;
+
+fn main() {
+    let git_rev = match Command::new("git").args(&["rev-parse", "HEAD"]).output() {
+        Ok(output) => String::from_utf8(output.stdout).unwrap(),
+        Err(_) => String::from("git-not-found"),
+    };
+    println!("cargo:rustc-env=GIT_REV={}", git_rev);
+}