optimize and refactor: read_to_string and read_i[u]64_from

There's so many Duplicated read_string_from method in different
subsystem's implementation, so as read_u64_from/read_i64_from methods.
(1) Move the read_string_from method into `lib.rs`,
called by each subsystem implementation as needed.
(2) Refactor read_u[i]64_from method with the help Rust Generic f
unction `read_from` and wrapped by read_u64_from or read_i64_from.

fix: #20

Signed-off-by: LiYa'nan <oliverliyn@gmail.com>

src/net_cls.rs

@@ -7,13 +7,13 @@
 //!
 //! See the Kernel's documentation for more information about this subsystem, found at:
 //!  [Documentation/cgroup-v1/net_cls.txt](https://www.kernel.org/doc/Documentation/cgroup-v1/net_cls.txt)
-use std::fs::File;
-use std::io::{Read, Write};
+use std::io::Write;
 use std::path::PathBuf;
 
 use crate::error::ErrorKind::*;
 use crate::error::*;
 
+use crate::read_u64_from;
 use crate::{
     ControllIdentifier, ControllerInternal, Controllers, NetworkResources, Resources, Subsystem,
 };
@@ -74,17 +74,6 @@ impl<'a> From<&'a Subsystem> for &'a NetClsController {
     }
 }
 
-fn read_u64_from(mut file: File) -> Result<u64> {
-    let mut string = String::new();
-    match file.read_to_string(&mut string) {
-        Ok(_) => string
-            .trim()
-            .parse()
-            .map_err(|e| Error::with_cause(ParseError, e)),
-        Err(e) => Err(Error::with_cause(ReadFailed, e)),
-    }
-}
-
 impl NetClsController {
     /// Constructs a new `NetClsController` with `root` serving as the root of the control group.
     pub fn new(root: PathBuf) -> Self {