# Which issue does this PR close?

small optimization

# Rationale for this change
key insight is the byte clone is cheap just a ref count compare to vec
clone is a alloc + memcopy.

before
```
let mut result = Vec::new();          // alloc #1
result.extend_from_slice(prefix);
result.extend_from_slice(suffix);

let data = Bytes::from(result.clone()); // alloc #2 + memcpy
item.set_from_bytes(data);
self.previous_value = result;          // keep Vec
```

after
```
let mut result = Vec::with_capacity(prefix_len + suffix.len()); // alloc #1
result.extend_from_slice(&self.previous_value[..prefix_len]);
result.extend_from_slice(suffix);

let data = Bytes::from(result);       // no alloc, takes Vec buffer
item.set_from_bytes(data.clone());    // cheap refcount bump
self.previous_value = data;           // move, no alloc
```

# What changes are included in this PR?
previous_value type changed to Bytes
preallocate result vec capacity.

# Are these changes tested?

the existing test should pass

# Are there any user-facing changes?

no