Add optional feature to not set 'fuzzing' flag

[Lukasz Rubaszewski]

Some crates (eg. secp256k1) stub some functions, when they detect 'fuzzing' flag, which makes them useless when fuzzing.

Idea is to add optional feature, that will prevent from setting fuzzing flag.

[Samuel Moelius]

Hey, @lrubasze. Okay if we close this?

[Lukasz Rubaszewski]

Hi @smoelius. Of course :)