feat: Add CellMap

[charliethomson]

This adds a trait CellMap<T> that allows you to test a predicate on the internal value of a Cell<T>

Examples

// New
let cell = Cell::from("Hello World!".to_owned());
assert!(cell.map_(|s| s.contains("Hello"));
assert_eq!(cell.take(), "Hello World!");

// Old
let cell = Cell::from("Hello World!".to_owned());
let s = cell.take();
assert!(s.contains("Hello"));
cell.set(s);
assert_eq!(cell.take(), "Hello World!");

This also changes a few tests to use this interface, test_condputs and test_number

[pickfire]

IMHO I don't think it is worth it, we could not utilize assert_eq! with this. Most of the cases, we could just cell.take() but we didn't since we want to keep the output, we could do that as well.

let cell = Cell::from("Hello World!".to_owned());
let s = cell.take();
assert!(s.contains("Hello"));
cell.set(s);
assert_eq!(cell.take(), "Hello World!");

It could be:

let cell = Cell::from("Hello World!".to_owned());
let s = cell.take();
assert!(s.contains("Hello"));
assert_eq!(s, "Hello World!");
assert_eq!(cell.take(), ""); // not tested

[pickfire]

@charliethomson Can we close this? I don't see much benefit this helper provides, it helps only a bit but makes it not ergonomic (_map).

[pickfire]

@charliethomson I am sorry about it. Thanks for the patch. :D