Support matrices of independent parameter lists

This patch permits the `-L`/`--parameter-list` argument to appear
multiple times. Each additional parameter increases the dimensionality
of the parameter matrix. One benchmark will be run for each combination
of parameters (i.e., the benchmark space is the Cartesian product of the
parameter spaces).

For now, `--parameter-list` and `--parameter-scan` are still mutually
exclusive. If desired, a follow-up change could similarly permit
multiple occurrences of `--parameter-scan` and also permit use of both
listed and scanned parameters together. (After all, `--parameter-scan`
can be thought of as syntactic sugar for a parameter list.)

This implementation is a little profligate with memory usage for
`BenchmarkResult`s: each result contains a separate copy of all
parameter names (as `String`s). This could be done away with by
threading lifetimes through the `BenchmarkResult`s, or by ref-counting
the names.

Because `BenchmarkResult`s now have maps with arbitrary key sets, we can
no longer use `serde` to serialize them to CSV. But a CSV writer is easy
to write by hand, so we just do that.

Fixes #253.

Test Plan:
Some unit tests included. As an integration test, try:

```
cargo run --release -- --runs 10 \
    -L foo foo1,foo2 -L bar bar9,bar8 \
    'echo {foo} {bar}' \
    'printf "%s\n" {foo} {bar}' \
    --export-csv /tmp/out
```

with all the export formats.

wchargin-branch: param-list-matrix
wchargin-source: 11051ab222c9de0fbc6ac6a080fbca48e05996c2

src/hyperfine/benchmark.rs

@@ -223,12 +223,7 @@ pub fn run_benchmark(
         } else {
             &values[num]
         };
-        match cmd.get_parameter() {
-            Some((param, value)) => {
-                Command::new_parametrized(preparation_command, param, value.clone())
-            }
-            None => Command::new(preparation_command),
-        }
+        Command::new_parametrized(preparation_command, cmd.get_parameters().clone())
     });
 
     // Warmup phase
@@ -414,16 +409,9 @@ pub fn run_benchmark(
     }
 
     // Run cleanup command
-    let cleanup_cmd =
-        options
-            .cleanup_command
-            .as_ref()
-            .map(|cleanup_command| match cmd.get_parameter() {
-                Some((param, value)) => {
-                    Command::new_parametrized(cleanup_command, param, value.clone())
-                }
-                None => Command::new(cleanup_command),
-            });
+    let cleanup_cmd = options.cleanup_command.as_ref().map(|cleanup_command| {
+        Command::new_parametrized(cleanup_command, cmd.get_parameters().clone())
+    });
     run_cleanup_command(&options.shell, &cleanup_cmd, options.show_output)?;
 
     Ok(BenchmarkResult::new(
@@ -436,6 +424,9 @@ pub fn run_benchmark(
         t_min,
         t_max,
         times_real,
-        cmd.get_parameter().as_ref().map(|p| p.1.to_string()),
+        cmd.get_parameters()
+            .iter()
+            .map(|(name, value)| ((*name).to_string(), value.to_string()))
+            .collect(),
     ))
 }