Use external, custom time function for no_std environments

[bitwave]

Added the possibility to create a custom current_millis function for no_std environments (e.g. custom OS, microcontroller, etc.)

[domenukk]

The no_std build now needs a default implementation (for [cfg(unix)] we can use libc, for the rest - probably just return 1 all the time, or a global variable that counts up on each call?)
Similarly, the no_std testcases need... some solution. Hard code 1?

[bitwave]

Similar to this code, we can use an external fn:

    time::Duration::from_millis(1)

[bitwave]

Can you list me the points where this needs to be implemented?

[domenukk]

Likely we need one inside a #[cfg(test)] in libafl, plus another one in the no_std testcase.
Btw. we could consider a more self-explanotary function name, like current_millis_no_std or something? Maybe @andreafioraldi has an opinion here

[bitwave]

Additionally we should provide documentation in docs/ folder how to do this?

[domenukk]

A .md file about using libafl in no_std environments would be awesome!