Provide a way to get marker strings in to a binary without the `fuzz!` macro

[smoelius]

From #499 (comment):

... a target binary must contain certain strings for AFL++ to handle it properly:

afl.rs/afl/src/lib.rs

Lines 49 to 52 in 462eff5

	// this marker strings needs to be in the produced executable for
	// afl-fuzz to detect `persistent mode` and `defered mode`
	static PERSIST_MARKER: &str = "##SIG_AFL_PERSISTENT##\0";
	static DEFERED_MARKER: &str = "##SIG_AFL_DEFER_FORKSRV##\0";

...

It used to be one could simply add the following to get those strings into their binary:

#[allow(unused_imports)]
use afl::fuzz;

... afl.rs should provide an easy way to get those strings into a binary.