Changes to Cargo: --bin by default

[steveklabnik]

So rust-lang/cargo#5029 is going to change the default for Cargo. We should make sure the book is accurate. Argh...

[carols10cents]

$ rg "cargo new" src
src/appendix-04-macros.md
217:$ cargo new hello-world
271:$ cargo new hello-world-derive
460:directory with `cargo new --bin pancakes`. We need to add both `hello-world`

src/ch01-02-hello-world.md
270:$ cargo new hello_cargo --bin
274:We passed the `--bin` argument to `cargo new` because our goal is to make an
284:file. Git is a common version control system. You can change `cargo new` to use
286:`--vcs` flag. Run `cargo new --help` to see the available options.

src/ch02-00-guessing-game-tutorial.md
21:$ cargo new guessing_game --bin
25:The first command, `cargo new`, takes the name of the project (`guessing_game`)
46:As you saw in Chapter 1, `cargo new` generates a “Hello, world!” program for

src/ch03-01-variables-and-mutability.md
11:*variables* in your *projects* directory by using `cargo new --bin variables`.

src/ch07-01-mod-and-the-filesystem.md
18:$ cargo new communicator

src/ch11-01-writing-tests.md
39:$ cargo new adder
60:automatically by `cargo new`</span>

src/ch12-01-accepting-command-line-arguments.md
3:Let’s create a new project with, as always, `cargo new`. We’ll call our project
8:$ cargo new --bin minigrep
22:Right now, the program generated by `cargo new` cannot process arguments we

src/ch14-02-publishing-to-crates-io.md
424:With a unique name, the version, the author details that `cargo new` added

src/ch14-03-cargo-workspaces.md
41:Next, we’ll create the `adder` binary crate by running `cargo new` within the
45:$ cargo new --bin adder
92:$ cargo new add-one

src/ch20-01-single-threaded.md
29:$ cargo new hello --bin

[carols10cents]

Looks like the spots we need to change to explicitly pass --lib are:

src/appendix-04-macros.md
217:$ cargo new hello-world
271:$ cargo new hello-world-derive

src/ch07-01-mod-and-the-filesystem.md
18:$ cargo new communicator

src/ch11-01-writing-tests.md
39:$ cargo new adder

src/ch14-03-cargo-workspaces.md
92:$ cargo new add-one

We should leave the --bin places the way they are to continue working with Rust 1.21.0 through the version that --bin becomes the default.