fix: replace unmaintained safemem with std alternatives

- [RUSTSEC-2023-0081](https://rustsec.org/advisories/RUSTSEC-2023-0081.html)
- rpgp/rpgp#300

Cargo.toml

@@ -11,7 +11,6 @@ readme = "README.md"
 edition = "2021"
 
 [dependencies]
-safemem = "0.3"
 memchr = "2.0"
 
 # `slice_deque` is only supported on platforms with virtual memory

src/buffer/std_buf.rs

@@ -66,7 +66,8 @@ impl StdBuf {
         // simply move the bytes down to the beginning
         let len = self.len();
 
-        safemem::copy_over(unsafe { self.buf.as_mut_slice() }, self.pos, 0, len);
+        let buf_slice = unsafe { self.buf.as_mut_slice() };
+        buf_slice.copy_within(self.pos..self.pos+len, 0);
 
         self.pos = 0;
         self.end = len;

src/lib.rs

@@ -1037,7 +1037,7 @@ impl Buffer {
         if self.zeroed < cap {
             unsafe {
                 let buf = self.buf.write_buf();
-                safemem::write_bytes(buf, 0);
+                buf.fill(0);
             }
 
             self.zeroed = cap;