support append for memory region

[my-vegetable-has-exploded]

async-rdma memory region need to use with lock_utilities::MappedRwLockReadGuard.

lmr.as_mut_slice().write(&[1_u8; 8])

I want to append(this can be done by wrapping with std::io::cursor) some bytes to mr to avoid useless copy.

But we can't build a cursor with (MappedRwLockReadGuard), like

    let cursor = std::io::Cursor::new(lmr.as_mut_slice());
	cursor.write(&[1_u8; 8])?;

so, i think we can consider providing a MappedRwLockWriteGuard<Cursor<&mut [u8]>>.

If needed, i can open a pr to add it.

[GTwhy]

Hi @my-vegetable-has-exploded , lmr.as_mut_slice() will return a MappedRwLockWriteGuard but not a MappedRwLockReadGuard. Do You mean the cursor you get by std::io::Cursor::new(lmr.as_mut_slice()) has a type of Cursor<MappedRwLockWriteGuard<&mut [u8]>>, which is not able to write, so you want to add an API to return a MappedRwLockWriteGuard<Cursor<&mut [u8]>>?

[my-vegetable-has-exploded]

yes
In this way, we can provide a convenient way for users to append to lmr without having to manage the cursor themselves.

[GTwhy]

@my-vegetable-has-exploded OK，welcome to open the PR!