[thin_shrink] Write thinp xml format reader and writer.

Still need to tidy up the error handling.

src/bin/thin_shrink.rs

@@ -0,0 +1,41 @@
+extern crate clap;
+extern crate thinp;
+
+use clap::{App, Arg};
+use std::process::exit;
+use thinp::file_utils;
+
+fn main() {
+    let parser = App::new("thin_shrink")
+	.version(thinp::version::TOOLS_VERSION)
+        .about("Rewrite xml metadata and move data in an inactive pool.")
+        .arg(Arg::with_name("INPUT")
+            .help("Specify thinp metadata xml file")
+            .required(true)
+            .long("input")
+            .value_name("INPUT")
+            .takes_value(true))
+        .arg(Arg::with_name("OUTPUT")
+            .help("Specify output xml file")
+            .required(true)
+            .long("output")
+            .value_name("OUTPUT")
+            .takes_value(true));
+
+    let matches = parser.get_matches();
+
+    // FIXME: check these look like xml
+    let input_file = matches.value_of("INPUT").unwrap();
+    let map_file = matches.value_of("MAP").unwrap();
+    let output_file = matches.value_of("OUTPUT").unwrap();
+
+    if !file_utils::file_exists(input_file) {
+        eprintln!("Couldn't find input file '{}'.", &input_file);
+        exit(1);
+    }
+
+    if let Err(reason) = thinp::shrink::toplevel::shrink(&input_file, &output_file) {
+        println!("Application error: {}\n", reason);
+        exit(1);
+    }
+}