Consider mark read_string as unsafe

[charlesxsh]

Hi there,

At line 77, read_string takes an address and used without validating/checking. If passing a invalid address, it might cause serious memory issues.From the library user point of view, it should not be causing any memory issue if only safe apis are used.

lurk/src/arch/mod.rs

Lines 70 to 79 in 719734e

	pub fn read_string(pid: Pid, address: c_ulonglong) -> String {
	let mut string = String::new();
	// Move 8 bytes up each time for next read.
	let mut count = 0;
	let word_size = 8;
	'done: loop {
	let address = unsafe { (address as *mut c_void).offset(count) };
	let res: c_long = match ptrace::read(pid, address) {