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/memory.rs

@@ -9,15 +9,14 @@
 //! See the Kernel's documentation for more information about this subsystem, found at:
 //!  [Documentation/cgroup-v1/memory.txt](https://www.kernel.org/doc/Documentation/cgroup-v1/memory.txt)
 use std::collections::HashMap;
-use std::fs::File;
-use std::io::{Read, Write};
+use std::io::Write;
 use std::path::PathBuf;
 use std::sync::mpsc::Receiver;
 
 use crate::error::ErrorKind::*;
 use crate::error::*;
 use crate::events;
-use crate::read_i64_from;
+use crate::{read_i64_from, read_string_from, read_u64_from};
 
 use crate::flat_keyed_to_hashmap;
 
@@ -870,25 +869,6 @@ impl<'a> From<&'a Subsystem> for &'a MemController {
     }
 }
 
-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)),
-    }
-}
-
-fn read_string_from(mut file: File) -> Result<String> {
-    let mut string = String::new();
-    match file.read_to_string(&mut string) {
-        Ok(_) => Ok(string.trim().to_string()),
-        Err(e) => Err(Error::with_cause(ReadFailed, e)),
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use crate::memory::{