Use libcore to memset a slice.

The API exists, we might as well use it.

src/blockdevice.rs

@@ -140,9 +140,7 @@ where
     /// Access a blank sector
     pub fn blank_mut(&mut self, block_idx: BlockIdx) -> &mut Block {
         self.block_idx = Some(block_idx);
-        for b in self.block[0].iter_mut() {
-            *b = 0;
-        }
+        self.block[0].fill(0);
         &mut self.block[0]
     }
 

src/sdcard/mod.rs

@@ -341,9 +341,7 @@ where
             return Err(Error::ReadError);
         }
 
-        for b in buffer.iter_mut() {
-            *b = 0xFF;
-        }
+        buffer.fill(0xFF);
         self.transfer_bytes(buffer)?;
 
         // These two bytes are always sent. They are either a valid CRC, or