Remove EM, Z from enter_target/leave_target

[tokatoka]

This is very important.
We need to fix this as soon as possible.
The bug in #2858 was caused by this.
Probably I'll do but i'll open here so that i don't forget

    #[inline]
    pub unsafe fn enter_target<EM, Z>(
        &mut self,
        fuzzer: &mut Z,
        state: &mut S,
        mgr: &mut EM,
        input: &I,
        executor_ptr: *const c_void,
    )

and

    /// This function marks the boundary between the fuzzer and the target
    #[inline]
    pub fn leave_target<EM, Z>(
        &mut self,
        _fuzzer: &mut Z,
        _state: &mut S,
        _mgr: &mut EM,
        _input: &I,
    )

EM and Z should be gone

I explained why this is bad in #2858 (comment)