Fix a compile error in derive(Deserialize) with no_std + alloc

[jplatte]

The fact that the existing no_std test suite only runs on Nightly (why is it a binary crate instead of a library crate?) and that there seems to be no test suite for the alloc feature at all makes me uncertain on how to add a test for this, but I'm happy to add it after being told where / how. This is it, anyway:

#![no_std]

use serde::Deserialize;

#[derive(Deserialize)]
struct Foo;

#[derive(Deserialize)]
struct Bar {
    #[serde(flatten)]
    foo: Foo,
}

Without this PR, it fails with this:

error[E0599]: no method named `to_string` found for type `&str` in the current scope
 --> src/lib.rs:8:10
  |
8 | #[derive(Deserialize)]
  |          ^^^^^^^^^^^
  |
  = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope, perhaps add a `use` for it:
  |
3 | use alloc::string::ToString;

[dtolnay]

Thanks!