replace unmaintained safemem::copy_over with slice::copy_within

* the `safemem` library is [unmaintained][1]
* both `safemem::copy_over` and `slice::copy_within` use `ptr::copy`
  internally, so the performance impact should me minimal.
* `slice::copy_within` was added in rust 1.37.0, but I don't know if
  lol-html has an official MSRV

[1]: https://rustsec.org/advisories/RUSTSEC-2023-0081.html

Cargo.toml

@@ -41,7 +41,6 @@ encoding_rs = "0.8.13"
 lazycell = "1.3.0"
 lazy_static = "1.3.0"
 memchr = "2.1.2"
-safemem = "0.3.3"
 selectors = "0.22.0"
 thiserror = "1.0.2"
 hashbrown = "0.13.1"

src/memory/arena.rs

@@ -1,5 +1,4 @@
 use super::{MemoryLimitExceededError, SharedMemoryLimiter};
-use safemem::copy_over;
 
 /// Preallocated region of memory that can grow and never deallocates during the lifetime of
 /// the limiter.
@@ -48,10 +47,8 @@ impl Arena {
     }
 
     pub fn shift(&mut self, byte_count: usize) {
-        let remainder_len = self.data.len() - byte_count;
-
-        copy_over(&mut self.data, byte_count, 0, remainder_len);
-        self.data.truncate(remainder_len);
+        self.data.copy_within(byte_count.., 0);
+        self.data.truncate(self.data.len() - byte_count);
     }
 
     pub fn bytes(&self) -> &[u8] {