Auto merge of #31914 - bluss:copy-from-slice-everywhere, r=alexcrichton

Use .copy_from_slice() where applicable

.copy_from_slice() does the same job of .clone_from_slice(), but the
former is explicitly for Copy elements and calls `memcpy` directly, and
thus is it efficient without optimization too.

src/libstd/sys/common/wtf8.rs

@@ -341,7 +341,7 @@ impl Wtf8Buf {
                 Some((surrogate_pos, _)) => {
                     pos = surrogate_pos + 3;
                     self.bytes[surrogate_pos..pos]
-                        .clone_from_slice(UTF8_REPLACEMENT_CHARACTER);
+                        .copy_from_slice(UTF8_REPLACEMENT_CHARACTER);
                 },
                 None => return unsafe { String::from_utf8_unchecked(self.bytes) }
             }