Add use declarations for std features.

Merge the `use` parts of the `no_std` branch. This reduces the diffs
between master and the `no_std` branch, making it easier to maintain.

Most of these changes are derived from patches by @lachlansneff in
https://github.com/Cretonne/cretonne/tree/no_std.

lib/cretonne/src/ir/jumptable.rs

@@ -8,6 +8,7 @@ use ir::entities::Ebb;
 use std::iter;
 use std::slice;
 use std::fmt::{self, Display, Formatter};
+use std::vec::Vec;
 
 /// Contents of a jump table.
 ///
@@ -141,6 +142,8 @@ mod tests {
     use super::JumpTableData;
     use ir::Ebb;
     use entity::EntityRef;
+    use std::vec::Vec;
+    use std::string::ToString;
 
     #[test]
     fn empty() {