Qualify uses of __fuzz! macro

[smoelius]

#161 introduced the __fuzz! macro, but did not refer to it with a qualified path. Consequently, users of afl.rs were forced to bring afl::__fuzz into their namespace, e.g., with use afl::*. This PR qualifies the uses of __fuzz! so that such a practice is no longer necessary.

For example, the following programs now compile, whereas they wouldn't before:

fn main() {
    afl::fuzz!(|data: &[u8]| {});
}

use afl::fuzz;

fn main() {
    fuzz!(|data: &[u8]| {});
}

[alex]

I think using $crate would be slightly more idiomatic

[smoelius]

I think using $crate would be slightly more idiomatic

I completely agree!

For others who weren't aware of this feature, it's described here: https://doc.rust-lang.org/reference/macros-by-example.html#hygiene