Use std::fs read/write conveniences

In addition to being more ergonomic these are much more efficient at reading
large files as they preallocate internally. This provides a nice speed boost
locally, reducing the overhead of `wasm-bindgen-test-runner` from 0.23s to
0.19s, yay!

crates/cli-support/src/lib.rs

@@ -13,8 +13,7 @@ extern crate failure;
 use std::any::Any;
 use std::collections::BTreeSet;
 use std::fmt;
-use std::fs::File;
-use std::io::{Read, Write};
+use std::fs;
 use std::mem;
 use std::path::{Path, PathBuf};
 
@@ -158,9 +157,7 @@ impl Bindgen {
                 (module, &name[..])
             }
             Input::Path(ref path) => {
-                let mut contents = Vec::new();
-                File::open(&path)
-                    .and_then(|mut f| f.read_to_end(&mut contents))
+                let contents = fs::read(&path)
                     .with_context(|_| format!("failed to read `{}`", path.display()))?;
                 let module = parity_wasm::deserialize_buffer::<Module>(&contents)
                     .context("failed to parse input file as wasm")?;
@@ -232,14 +229,12 @@ impl Bindgen {
 
         let extension = if self.nodejs_experimental_modules { "mjs" } else { "js" };
         let js_path = out_dir.join(stem).with_extension(extension);
-        File::create(&js_path)
-            .and_then(|mut f| f.write_all(reset_indentation(&js).as_bytes()))
+        fs::write(&js_path, reset_indentation(&js))
             .with_context(|_| format!("failed to write `{}`", js_path.display()))?;
 
         if self.typescript {
             let ts_path = out_dir.join(stem).with_extension("d.ts");
-            File::create(&ts_path)
-                .and_then(|mut f| f.write_all(ts.as_bytes()))
+            fs::write(&ts_path, ts)
                 .with_context(|_| format!("failed to write `{}`", ts_path.display()))?;
         }
 
@@ -248,14 +243,12 @@ impl Bindgen {
         if self.nodejs {
             let js_path = wasm_path.with_extension(extension);
             let shim = self.generate_node_wasm_import(&module, &wasm_path);
-            File::create(&js_path)
-                .and_then(|mut f| f.write_all(shim.as_bytes()))
+            fs::write(&js_path, shim)
                 .with_context(|_| format!("failed to write `{}`", js_path.display()))?;
         }
 
         let wasm_bytes = parity_wasm::serialize(module)?;
-        File::create(&wasm_path)
-            .and_then(|mut f| f.write_all(&wasm_bytes))
+        fs::write(&wasm_path, wasm_bytes)
             .with_context(|_| format!("failed to write `{}`", wasm_path.display()))?;
         Ok(())
     }

crates/cli-support/src/wasm2es6js.rs

@@ -2,8 +2,8 @@ extern crate base64;
 extern crate tempfile;
 
 use std::collections::{HashMap, HashSet};
-use std::fs::File;
-use std::io::{self, Read, Write};
+use std::fs;
+use std::io;
 use std::process::Command;
 
 use failure::{Error, ResultExt};
@@ -375,8 +375,7 @@ impl Output {
         let td = tempfile::tempdir()?;
         let wasm = serialize(self.module)?;
         let wasm_file = td.as_ref().join("foo.wasm");
-        File::create(&wasm_file)
-            .and_then(|mut f| f.write_all(&wasm))
+        fs::write(&wasm_file, wasm)
             .with_context(|_| format!("failed to write wasm to `{}`", wasm_file.display()))?;
 
         let wast_file = td.as_ref().join("foo.wast");
@@ -396,9 +395,7 @@ impl Output {
             "wasm2asm",
         )?;
 
-        let mut asm_func = String::new();
-        File::open(&js_file)
-            .and_then(|mut f| f.read_to_string(&mut asm_func))
+        let asm_func = fs::read_to_string(&js_file)
             .with_context(|_| format!("failed to read `{}`", js_file.display()))?;
 
         let mut make_imports = String::from(