Inline literals in generated code

Part of the Debug example currently compiles to:

```rust
let __v7 = "x";
let _ = ::std::fmt::DebugStruct::field(__v6, __v7, __v3);
```

It doesn't matter here for correctness but this would be more readable in `cargo expand` if emitted as:

```rust
let _ = ::std::fmt::DebugStruct::field(__v6, "x", __v3);
```