feat(bolero-generator): add support for calling generators from anywhere

[camshaft]

This change introduces the bolero_generator::any module, which enables calling generators from anywhere in the program.

use bolero_generator::prelude::*;

let value: u64 = any();
let value: u64 = (0..3).any();
let value = gen::<Vec<u8>>().any();
let value = gen::<Vec<u8>>().with().len(2..4).any();