Implement From and FromIterator for widgets::plot::Value(s)

[KmolYuan]

• Implement From for widgets::plot::Value, for [f64; 2], (f64, f64) and their valid type.
• Implement FromIterator for widgets::plot::Values, for any iterator containing Value type.
• Use IntoIterator to support the types that can deconstruct themselves.

This PR makes the following code work:

use egui::widgets::plot::Values;

let v: Vec<MyType>;
let values = v.iter()
    // do the iterator things
    .map(|my| [my.x(), my.y()])
    .collect::<Values>();

Before:

use egui::widgets::plot::{Value, Values};

let v: Vec<MyType>;
let iter = v.iter()
    // do the iterator things
    .map(|my| Value::new(my.x(), my.y()));
let values = Values::from_values_iter(iter);

[EmbersArc]

Hey, have a look at #1816 which adds this amongst other small improvements. Feel free to make suggestions there.

[emilk]

Is this still relevant after #1816 ?

[KmolYuan]

@emilk This PR is fully covered in #1816.