use top level fs functions where appropriate

This commit replaces many usages of `File::open` and reading or writing
with `fs::read_to_string`, `fs::read` and `fs::write`. This reduces code
complexity, and will improve performance for most reads, since the
functions allocate the buffer to be the size of the file.

I believe that this commit will not impact behavior in any way, so some
matches will check the error kind in case the file was not valid UTF-8.
Some of these cases may not actually care about the error.

src/libsyntax/source_map.rs

@@ -31,7 +31,7 @@ use std::path::{Path, PathBuf};
 
 use std::env;
 use std::fs;
-use std::io::{self, Read};
+use std::io;
 use errors::SourceMapper;
 
 /// Return the span itself if it doesn't come from a macro expansion,
@@ -96,9 +96,7 @@ impl FileLoader for RealFileLoader {
     }
 
     fn read_file(&self, path: &Path) -> io::Result<String> {
-        let mut src = String::new();
-        fs::File::open(path)?.read_to_string(&mut src)?;
-        Ok(src)
+        fs::read_to_string(path)
     }
 }