Initial work on tracking memory usage

src/benchmark/benchmark_result.rs

@@ -42,6 +42,10 @@ pub struct BenchmarkResult {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub times: Option<Vec<Second>>,
 
+    /// Maximum memory usage of the process, in bytes
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub memory_usage_byte: Option<Vec<u64>>,
+
     /// Exit codes of all command invocations
     pub exit_codes: Vec<Option<i32>>,