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/partition_slice.rs

@@ -33,6 +33,7 @@ where
 #[cfg(test)]
 mod tests {
     use super::partition_slice;
+    use std::vec::Vec;
 
     fn check(x: &[u32], want: &[u32]) {
         assert_eq!(x.len(), want.len());