Function process 

pub fn process<'ctx, L, F>(
    ctx: &mut ReportContext<'ctx>,
    loader: L,
    options: &ProcessOptions,
) -> Result<Ledger<'ctx>, ReportError>
where
    L: Borrow<Loader<F>>,
    F: FileSystem,

Takes the loader, and gives back the all read transactions. Also returns the computed balance, as a side-artifact. Usually this needs to be reordered, so just returning a Vec.