Allow multiple imports in a single statement

Like so: import foo::{bar, baz};

Issue #817

src/test/run-pass/import-from-native.rs

@@ -0,0 +1,15 @@
+mod spam {
+    fn ham() {}
+    fn eggs() {}
+}
+
+native "rust" mod rustrt {
+    import spam::{ham, eggs};
+    export ham;
+    export eggs;
+}
+
+fn main() {
+    rustrt::ham();
+    rustrt::eggs();
+}

src/test/run-pass/import-from.rs

@@ -0,0 +1,11 @@
+import spam::{ham, eggs};
+
+mod spam {
+    fn ham() {}
+    fn eggs() {}
+}
+
+fn main() {
+    ham();
+    eggs();
+}