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/isa/riscv/mod.rs

@@ -14,6 +14,7 @@ use isa::{TargetIsa, RegInfo, RegClass, EncInfo};
 use ir;
 use regalloc;
 use std::fmt;
+use std::boxed::Box;
 
 #[allow(dead_code)]
 struct Isa {
@@ -116,6 +117,7 @@ mod tests {
     use isa;
     use ir::{DataFlowGraph, InstructionData, Opcode};
     use ir::{types, immediates};
+    use std::string::{String, ToString};
 
     fn encstr(isa: &isa::TargetIsa, enc: Result<isa::Encoding, isa::Legalize>) -> String {
         match enc {