add file_name field to Mollusk struct

[AkliluYirgalem]

groundwork for multi-program benchmarking: Added a file_name field to Mollusk Struct. This allows the bencher crate to track individual program filename, which i will use in the next PR to implement multi-program benchmarking and the filename will be used in reporting the Markdown file.

[buffalojoec]

This relationship is a little misleading if we put the file name on the Mollusk struct. A Mollusk instance houses a program cache, which can hold many programs and be reused to invoke multiple programs at the top-level, especially over transaction contexts and instruction chains.

Is there a way we can refactor the bencher to support file name distinguishing instead?

[AkliluYirgalem]

Yes, you are right, that was an oversimplification of naming a program, i immediately thought of a list of file names but thats also an overcomplicating solution.

about refactoring the bencher, i can think of two solutions, one is accepting a generic param from the new function and converting their types using the From trait(by implementing it in Mollusk), into Vec of tuples, like (mollusk, file_name), that will preserve backward compatibility by giving default file_name for uninformed tests.
the other is by just accepting a vector of tuples, much like bench, this will break old tests.

a snapshot for the first one:

//for backward compatability
impl From<Mollusk> for Vec<(Mollusk, String)> {
    fn from(m: Mollusk) -> Self {
        vec![(m, String::new("default_file_name"))] 
    }
}

//for the newer inputs
impl From<(Mollusk, &str)> for Vec<(Mollusk, String)> {
    fn from((m, s): (Mollusk, &str)) -> Self {
        vec![(m, s.to_string())]
    }
}