This is the recommended best practice for binaries, we should do it for
`cargo-3ds` as well. A side effect of this is that we should test build
and installation with the exact Cargo.lock dependencies with `--locked`.

Use `cargo config get` to check if the runner is configured, and if so
use it like a normal `cargo run` invocation would. Otherwise, fall back
to the default `3dslink` behavior we had before.

Minor tweaks like fn names, refactoring a little etc.

Also update README to include an install command, with `--locked` as
well.