Strictly typed dataframes using macros

[abhemanyus]

The current architecture hides data types behind a generic interface. The resultant code is cumbersome to work with, and bereft of type information that should ideally be present.
Since most data-processing tasks will be performed on data with a known structure, providing an API that can make effective use of it will greatly improve the user experience.

Usage example:

#[fisher_rs::Table]
struct Citizen {
  name: String,
  age: u32,
  country: String
}

let citizens = Citizen::load_csv("citizens.csv");
let mean_age: f32 = citizens.age.mean();
let first_citizen = citizens.first().unwrap();

The implementation can provide various views into the internal data. Collective: citizens.age; Individual: citizens.first().
Lazy loading can be implemented for extremely large datasets.
Using views, external libraries can be provided data in suitable structures.