Minimize unsafety in encode_utf8

Use slice patterns to avoid having to skip bounds checking

src/libcore/lib.rs

@@ -129,6 +129,7 @@
 #![feature(associated_type_bounds)]
 #![feature(const_type_id)]
 #![feature(const_caller_location)]
+#![feature(slice_patterns)]
 
 #[prelude_import]
 #[allow(unused)]