Mark memset function as unsafe

[kitcatier]

Hello, I found a soundness issue in this crate.

ckb-vm/src/memory/mod.rs

Lines 131 to 138 in 1642d8f

	// Keep this in a central place to allow for future optimization
	#[inline(always)]
	pub fn memset(slice: &mut [u8], value: u8) {
	let p = slice.as_mut_ptr();
	unsafe {
	ptr::write_bytes(p, value, slice.len());
	}
	}

The unsafe function called needs to ensure that the parameter must be:

• dst must be valid for writes of count * size_of::() bytes.
• dst must be properly aligned.
  https://doc.rust-lang.org/std/ptr/fn.write_bytes.html
  and the developer who calls the ref_as_nonnull function may not notice this safety requirement.
  Marking them unsafe also means that callers must make sure they know what they're doing.