expose HYPERFINE_ITERATION to intermediate commands

Fixes: #781

Expose the `$HYPERFINE_ITERATION` variable to intermediate commands.
This will allow iteration-specific prepare/conclude, such as:

```bash
hyperfine --runs 5 --prepare 'echo touch file${HYPERFINE_ITERATION}.dat' \
    'echo testing file${HYPERFINE_ITERATION}.dat' \
    --conclude 'echo rm file${HYPERFINE_ITERATION}.dat' --show-output
```

src/benchmark/executor.rs

@@ -36,7 +36,7 @@ pub trait Executor {
     fn run_command_and_measure(
         &self,
         command: &Command<'_>,
-        iteration: BenchmarkIteration,
+        iteration: &BenchmarkIteration,
         command_failure_action: Option<CmdFailureAction>,
         output_policy: &CommandOutputPolicy,
     ) -> Result<Measurement>;
@@ -56,7 +56,7 @@ pub trait Executor {
 
 fn run_command_and_measure_common(
     mut command: std::process::Command,
-    iteration: BenchmarkIteration,
+    iteration: &BenchmarkIteration,
     command_failure_action: CmdFailureAction,
     command_input_policy: &CommandInputPolicy,
     command_output_policy: &CommandOutputPolicy,
@@ -132,7 +132,7 @@ impl Executor for RawExecutor<'_> {
     fn run_command_and_measure(
         &self,
         command: &Command<'_>,
-        iteration: BenchmarkIteration,
+        iteration: &BenchmarkIteration,
         command_failure_action: Option<CmdFailureAction>,
         output_policy: &CommandOutputPolicy,
     ) -> Result<Measurement> {
@@ -175,7 +175,7 @@ impl Executor for ShellExecutor<'_> {
     fn run_command_and_measure(
         &self,
         command: &Command<'_>,
-        iteration: BenchmarkIteration,
+        iteration: &BenchmarkIteration,
         command_failure_action: Option<CmdFailureAction>,
         output_policy: &CommandOutputPolicy,
     ) -> Result<Measurement> {
@@ -230,7 +230,7 @@ impl Executor for ShellExecutor<'_> {
             // Just run the shell without any command
             let res = self.run_command_and_measure(
                 &Command::new(None, ""),
-                BenchmarkIteration::NonBenchmarkRun,
+                &BenchmarkIteration::NonBenchmarkRun,
                 None,
                 &CommandOutputPolicy::Null,
             );
@@ -304,7 +304,7 @@ impl Executor for MockExecutor {
     fn run_command_and_measure(
         &self,
         command: &Command<'_>,
-        _iteration: BenchmarkIteration,
+        _iteration: &BenchmarkIteration,
         _command_failure_action: Option<CmdFailureAction>,
         _output_policy: &CommandOutputPolicy,
     ) -> Result<Measurement> {