Function glassbench::pretend_used

pub fn pretend_used<T>(t: T) -> T

tell the compiler not to optimize away the given argument (which is expected to be the function call you want to benchmark).

This should use core::hint::bench_black_box but it’s not yet stabilized, see https://github.com/rust-lang/rust/issues/64102

In the meantime, it uses the same implementation than Criterion.