Be more consistent about tests vs. main (#2644)

The content slides all use `fn main`, with the exception of the testing
segment. But with this change, where it makes sense exercises use tests
instead, and not both tests and `fn main`.

A small change in `book.js` supports running tests when a code sample
does not have `fn main` but does have `#[test]`, so these work
naturally.

Fixes #1581.

src/iterators/Cargo.toml

@@ -4,6 +4,6 @@ version = "0.1.0"
 edition = "2021"
 publish = false
 
-[[bin]]
-name = "offset-differences"
+[lib]
+name = "offset_differences"
 path = "exercise.rs"

src/iterators/exercise.md

@@ -11,7 +11,7 @@ Copy the following code to <https://play.rust-lang.org/> and make the tests
 pass. Use an iterator expression and `collect` the result to construct the
 return value.
 
-```rust
+```rust,editable
 {{#include exercise.rs:offset_differences}}
     todo!()
 }

src/iterators/exercise.rs

@@ -51,5 +51,3 @@ fn test_degenerate_cases() {
     assert_eq!(offset_differences(1, empty), vec![]);
 }
 // ANCHOR_END: unit-tests
-
-fn main() {}