Make Errno::result() inline

This makes the success case a single `cmpl $-1, %eax` followed
with a non-taken conditional jump, instead of the current `callq`
which is always taken.

I have not benchmarked the difference, but it is an obvious
improvement.

Author: linkmauve

src/errno.rs

@@ -103,6 +103,7 @@ impl Errno {
 
     /// Returns `Ok(value)` if it does not contain the sentinel value. This
     /// should not be used when `-1` is not the errno sentinel value.
+    #[inline]
     pub fn result<S: ErrnoSentinel + PartialEq<S>>(value: S) -> Result<S> {
         if value == S::sentinel() {
             Err(Self::last())