Always inline run_utf8_validation.

It only has two call sites, and the one within `from_utf8` is hot within
rustc itself.

src/libcore/str/mod.rs

@@ -1499,7 +1499,7 @@ fn contains_nonascii(x: usize) -> bool {
 
 /// Walks through `v` checking that it's a valid UTF-8 sequence,
 /// returning `Ok(())` in that case, or, if it is invalid, `Err(err)`.
-#[inline]
+#[inline(always)]
 fn run_utf8_validation(v: &[u8]) -> Result<(), Utf8Error> {
     let mut index = 0;
     let len = v.len();