Add inline attribute to String::from_str_in (#290)

Saw that `String::from_str_in` pop up in my own application benchmark
profiling. It seems like a good candidate for inlining as
`String::with_capacity_in` is inlined and `String::from_str_in` isn't
doing much work other than that.

Running the bumpalo benchmarks, the from_str_in did not see any changes,
though my own benchmarks show a consistent 5-10% improvement in latency.

Author: nickbabcock

src/collections/string.rs

@@ -679,6 +679,7 @@ impl<'bump> String<'bump> {
     /// let s = String::from_str_in("hello", &b);
     /// assert_eq!(s, "hello");
     /// ```
+    #[inline]
     pub fn from_str_in(s: &str, bump: &'bump Bump) -> String<'bump> {
         let len = s.len();
         let mut t = String::with_capacity_in(len, bump);