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/condcodes.rs

@@ -266,6 +266,7 @@ impl FromStr for FloatCC {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::string::ToString;
 
     static INT_ALL: [IntCC; 10] = [
         IntCC::Equal,

lib/cretonne/src/ir/dfg.rs

@@ -944,6 +944,7 @@ mod tests {
     use cursor::{Cursor, FuncCursor};
     use ir::types;
     use ir::{Function, Opcode, InstructionData, TrapCode};
+    use std::string::ToString;
 
     #[test]
     fn make_inst() {

lib/cretonne/src/ir/entities.rs

@@ -262,6 +262,7 @@ impl From<Heap> for AnyEntity {
 mod tests {
     use super::*;
     use std::u32;
+    use std::string::ToString;
 
     #[test]
     fn value_with_number() {

lib/cretonne/src/ir/extfunc.rs

@@ -10,6 +10,7 @@ use isa::{RegInfo, RegUnit};
 use std::cmp;
 use std::fmt;
 use std::str::FromStr;
+use std::vec::Vec;
 
 /// Function signature.
 ///
@@ -378,6 +379,7 @@ impl FromStr for CallConv {
 mod tests {
     use super::*;
     use ir::types::{I32, F32, B8};
+    use std::string::ToString;
 
     #[test]
     fn argument_type() {

lib/cretonne/src/ir/extname.rs

@@ -122,6 +122,7 @@ impl FromStr for ExternalName {
 mod tests {
     use super::ExternalName;
     use ir::LibCall;
+    use std::string::ToString;
 
     #[test]
     fn display_testcase() {

lib/cretonne/src/ir/immediates.rs

@@ -652,6 +652,7 @@ mod tests {
     use std::{f32, f64};
     use std::str::FromStr;
     use std::fmt::Display;
+    use std::string::ToString;
 
     #[test]
     fn format_imm64() {

lib/cretonne/src/ir/instructions.rs

@@ -9,6 +9,7 @@
 use std::fmt::{self, Display, Formatter};
 use std::str::FromStr;
 use std::ops::{Deref, DerefMut};
+use std::vec::Vec;
 
 use ir;
 use ir::{Value, Type, Ebb, JumpTable, SigRef, FuncRef};
@@ -537,6 +538,7 @@ pub enum ResolvedConstraint {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::string::ToString;
 
     #[test]
     fn opcodes() {

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() {

lib/cretonne/src/ir/layout.rs

@@ -743,6 +743,7 @@ mod tests {
     use entity::EntityRef;
     use ir::{Ebb, Inst, ProgramOrder, SourceLoc};
     use std::cmp::Ordering;
+    use std::vec::Vec;
 
     struct LayoutCursor<'f> {
         /// Borrowed function layout. Public so it can be re-borrowed from this cursor.

lib/cretonne/src/ir/libcall.rs

@@ -100,6 +100,7 @@ impl LibCall {
 #[cfg(test)]
 mod test {
     use super::*;
+    use std::string::ToString;
 
     #[test]
     fn display() {

lib/cretonne/src/ir/progpoint.rs

@@ -148,6 +148,7 @@ mod tests {
     use super::*;
     use entity::EntityRef;
     use ir::{Inst, Ebb};
+    use std::string::ToString;
 
     #[test]
     fn convert() {

lib/cretonne/src/ir/sourceloc.rs

@@ -51,6 +51,7 @@ impl fmt::Display for SourceLoc {
 #[cfg(test)]
 mod tests {
     use ir::SourceLoc;
+    use std::string::ToString;
 
     #[test]
     fn display() {

lib/cretonne/src/ir/stackslot.rs

@@ -10,6 +10,7 @@ use std::cmp;
 use std::fmt;
 use std::ops::{Index, IndexMut};
 use std::str::FromStr;
+use std::vec::Vec;
 
 /// The size of an object on the stack, or the size of a stack frame.
 ///
@@ -319,6 +320,7 @@ mod tests {
     use ir::Function;
     use ir::types;
     use super::*;
+    use std::string::ToString;
 
     #[test]
     fn stack_slot() {

lib/cretonne/src/ir/trapcode.rs

@@ -89,6 +89,7 @@ impl FromStr for TrapCode {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::string::ToString;
 
     // Everything but user-defined codes.
     const CODES: [TrapCode; 8] = [

lib/cretonne/src/ir/types.rs

@@ -324,6 +324,7 @@ impl Default for Type {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::string::ToString;
 
     #[test]
     fn basic_scalars() {