Add OCaml package manager (OPAM) (#62)

This adds support for the OCaml package manager (OPAM).

The command line `opam update` first performs a self-update if
necessary. Then it updates its package list.
Finally, `opam upgrade` upgrades all locally installed packages.

src/generic.rs

@@ -94,6 +94,23 @@ pub fn run_rustup(base_dirs: &BaseDirs, terminal: &mut Terminal, dry_run: bool)
     None
 }
 
+#[must_use]
+pub fn run_opam_update(terminal: &mut Terminal, dry_run: bool) -> Option<(&'static str, bool)> {
+    if let Some(opam) = utils::which("opam") {
+        terminal.print_separator("OCaml Package Manager");
+
+        let success = || -> Result<(), Error> {
+            Executor::new(&opam, dry_run).arg("update").spawn()?.wait()?.check()?;
+            Executor::new(&opam, dry_run).arg("upgrade").spawn()?.wait()?.check()?;
+            Ok(())
+        }().is_ok();
+
+        return Some(("OPAM", success));
+    }
+
+    None
+}
+
 #[must_use]
 pub fn run_custom_command(name: &str, command: &str, terminal: &mut Terminal, dry_run: bool) -> Result<(), Error> {
     terminal.print_separator(name);