Move slices and strings to references section (#1898)

This PR moves the slides for slices and strings into the day 1 section
on references. This seems like the more natural place to introduce
slices since slices are a type of reference. It then also made sense to
me to follow that with the introduction of `&str` and `String`, since
students now have the context to understand what a "string slice" is. I
also removed the strings slide from the types and values section since
it didn't make sense to cover the same topic twice in the same day. I
tested this new organization in my class on Wednesday and it didn't
cause day 1 to take too long.

src/types-and-values/strings.md

@@ -1,59 +0,0 @@
----
-minutes: 5
----
-
-# Strings
-
-Rust has two types to represent strings, both of which will be covered in more
-depth later. Both _always_ store UTF-8 encoded strings.
-
-- `String` - a modifiable, owned string.
-- `&str` - a read-only string. String literals have this type.
-
-```rust,editable
-fn main() {
-    let greeting: &str = "Greetings";
-    let planet: &str = "🪐";
-    let mut sentence = String::new();
-    sentence.push_str(greeting);
-    sentence.push_str(", ");
-    sentence.push_str(planet);
-    println!("final sentence: {}", sentence);
-    println!("{:?}", &sentence[0..5]);
-    //println!("{:?}", &sentence[12..13]);
-}
-```
-
-<details>
-
-This slide introduces strings. Everything here will be covered in more depth
-later, but this is enough for subsequent slides and exercises to use strings.
-
-- Invalid UTF-8 in a string is UB, and this not allowed in safe Rust.
-
-- `String` is a user-defined type with a constructor (`::new()`) and methods
-  like `s.push_str(..)`.
-
-- The `&` in `&str` indicates that this is a reference. We will cover references
-  later, so for now just think of `&str` as a unit meaning "a read-only string".
-
-- The commented-out line is indexing into the string by byte position. `12..13`
-  does not end on a character boundary, so the program panics. Adjust it to a
-  range that does, based on the error message.
-
-- Raw strings allow you to create a `&str` value with escapes disabled:
-  `r"\n" == "\\n"`. You can embed double-quotes by using an equal amount of `#`
-  on either side of the quotes:
-
-  <!-- mdbook-xgettext: skip -->
-  ```rust,editable
-  fn main() {
-      println!(r#"<a href="link.html">link</a>"#);
-      println!("<a href=\"link.html\">link</a>");
-  }
-  ```
-
-- Using `{:?}` is a convenient way to print array/vector/struct of values for
-  debugging purposes, and it's commonly used in code.
-
-</details>