Add option to export JSON, change file handling

No longer serialize into strings, instead use Vec<u8> as it is
what we're writing to the files.

Fixup comments

src/hyperfine/export/mod.rs

@@ -1,8 +1,11 @@
 mod csv;
+mod json;
 
 use self::csv::CsvExporter;
+use self::json::JsonExporter;
 
-use std::io::Result;
+use std::io::{Result, Write};
+use std::fs::File;
 
 use hyperfine::internal::Second;
 
@@ -51,16 +54,19 @@ impl ExportEntry {
 
 /// The ResultExportType enum is used to denote the desired form
 /// of exporter to use for a given file.
+#[derive(Clone)]
 pub enum ResultExportType {
     /// Export to a csv file with the provided name
     Csv(String),
+    /// Export to a JSON file
+    Json(String),
 }
 
 /// A ResultExporter is responsible for writing all results to the
 /// appropriate file
 trait ResultExporter {
     /// Write all entries to the target destination
-    fn write(&self, values: &Vec<ExportEntry>) -> Result<()>;
+    fn write(&self, values: &Vec<ExportEntry>) -> Result<Vec<u8>>;
 }
 
 /// Create a new ExportManager
@@ -70,23 +76,36 @@ pub fn create_export_manager() -> ExportManager {
     }
 }
 
-/// The Exporter is the internal implementation of the ExportManager
+/// The ExportManager handles the management of multiple file
+/// exporters.
 pub struct ExportManager {
-    exporters: Vec<Box<ResultExporter>>,
+    exporters: Vec<ResultExportType>,
 }
 
 impl ExportManager {
-    pub fn add_exporter(&mut self, for_type: &ResultExportType) {
-        match for_type {
-            &ResultExportType::Csv(ref file_name) => self.exporters
-                .push(Box::from(CsvExporter::new(file_name.clone()))),
-        };
+    /// Add an additional exporter to the ExportManager
+    pub fn add_exporter(&mut self, for_type: ResultExportType) {
+        self.exporters.push(for_type);
     }
 
+    /// Write the given results to all Exporters contained within this manager
     pub fn write_results(&self, to_write: Vec<ExportEntry>) -> Result<()> {
         for exp in &self.exporters {
-            exp.write(&to_write)?;
+            let (exporter, filename): (Box<ResultExporter>, &str) = match exp {
+                &ResultExportType::Csv(ref file) => (Box::from(CsvExporter::new()), file),
+                &ResultExportType::Json(ref file) => (Box::from(JsonExporter::new()), file),
+            };
+
+            let file_content = exporter.write(&to_write)?;
+            write_to_file(filename, file_content)?;
         }
         Ok(())
     }
 }
+
+/// Write the given content to a file with the specified name
+fn write_to_file(filename: &str, content: Vec<u8>) -> Result<()> {
+    let mut file = File::create(filename)?;
+    file.write_all(&content)?;
+    Ok(())
+}