Parse underscores in identifiers for format!

Closes #9119

src/libstd/fmt/parse.rs

@@ -554,7 +554,7 @@ impl<'self> Parser<'self> {
     /// characters.
     fn word(&mut self) -> &'self str {
         let start = match self.cur.clone().next() {
-            Some((pos, c)) if char::is_alphabetic(c) => {
+            Some((pos, c)) if char::is_XID_start(c) => {
                 self.cur.next();
                 pos
             }
@@ -563,7 +563,7 @@ impl<'self> Parser<'self> {
         let mut end;
         loop {
             match self.cur.clone().next() {
-                Some((_, c)) if char::is_alphanumeric(c) => {
+                Some((_, c)) if char::is_XID_continue(c) => {
                     self.cur.next();
                 }
                 Some((pos, _)) => { end = pos; break }

src/test/run-pass/ifmt.rs

@@ -82,6 +82,7 @@ pub fn main() {
     t!(format!("{foo} {1} {bar} {0}", 0, 1, foo=2, bar=3), "2 1 3 0");
     t!(format!("{} {0:s}", "a"), "a a");
     t!(format!("{} {0}", "a"), "a a");
+    t!(format!("{foo_bar}", foo_bar=1), "1");
 
     // Methods should probably work
     t!(format!("{0, plural, =1{a#} =2{b#} zero{c#} other{d#}}", 0u), "c0");