Prefer encoding the char when checking for string prefix/suffix

This enables constant folding when matching a literal char.

Fixes rust-lang#41993.

src/libcore/str/pattern.rs

@@ -445,21 +445,15 @@ impl<'a> Pattern<'a> for char {
 
     #[inline]
     fn is_prefix_of(self, haystack: &'a str) -> bool {
-        if let Some(ch) = haystack.chars().next() {
-            self == ch
-        } else {
-            false
-        }
+        let mut buffer = [0u8; 4];
+        self.encode_utf8(&mut buffer).is_prefix_of(haystack)
     }
 
     #[inline]
     fn is_suffix_of(self, haystack: &'a str) -> bool where Self::Searcher: ReverseSearcher<'a>
     {
-        if let Some(ch) = haystack.chars().next_back() {
-            self == ch
-        } else {
-            false
-        }
+        let mut buffer = [0u8; 4];
+        self.encode_utf8(&mut buffer).is_suffix_of(haystack)
     }
 }