StivaleTerminalTag::write is unsound

[chorman0773]

https://github.com/Andy-Python-Programmer/stivale/blob/master/src/v2/tag.rs#L88 indicates a safety comment of

This function is not thread safe

but it's applied to a safe by-shared-reference reciever method of a Sync type, making this unsound.

[chorman0773]

Also, as all of the fields of StivaleTerminalTag::write are public, a user could create such a tag with a 0 in the term_write_addr slot, and call term_write on it, causing immediate undefined behaviour by violating the validity invariant of extern"C" fn(*mut u8,u64) (null function pointer).