clippy: remove redundant closures

```
src/main.rs:216:62: 216:85 warning: redundant closure found, #[warn(redundant_closure)] on by default
src/main.rs:216                 symlink_root: m.value_of("symlink-root").map(|slr| String::from(slr)),
                                                                             ^~~~~~~~~~~~~~~~~~~~~~~
src/main.rs:216:62: 216:85 help: remove closure as shown:
src/main.rs:                    symlink_root: m.value_of("symlink-root").map(String::from),
src/main.rs:216:62: 216:85 help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#redundant_closure
```

Signed-off-by: Paul Osborne <[email protected]>

Author: posborne

src/main.rs

@@ -213,21 +213,21 @@ fn main() {
             let unexport_options = GpioUnexportOptions {
                 gpio_opts: gpio_options,
                 pin: String::from(m.value_of("pin").unwrap()),
-                symlink_root: m.value_of("symlink-root").map(|slr| String::from(slr)),
+                symlink_root: m.value_of("symlink-root").map(String::from),
             };
             gpio_unexport::main(&cfg, &unexport_options);
         }
         ("unexport-all", Some(m)) => {
             let unexportall_options = GpioUnexportAllOptions {
                 gpio_opts: gpio_options,
-                symlink_root: m.value_of("symlink-root").map(|slr| String::from(slr)),
+                symlink_root: m.value_of("symlink-root").map(String::from),
             };
             gpio_unexportall::main(&cfg, &unexportall_options);
         }
         ("status", Some(m)) => {
             let status_options = GpioStatusOptions {
                 gpio_opts: gpio_options,
-                pin: m.value_of("pin").map(|pin| String::from(pin)),
+                pin: m.value_of("pin").map(String::from),
             };
             gpio_status::main(&cfg, &status_options);
         }