Fixed compilation on rust 1.35.0

[ko1N]

The current version of cargo-eval does not compile anymore with the following error:

error[E0277]: the trait bound `std::string::String: std::convert::From<&std::string::String>` is not satisfied
   --> src/main.rs:247:28
    |
247 |             content = expr.into();
    |                            ^^^^ the trait `std::convert::From<&std::string::String>` is not implemented for `std::string::String`
    |
    = help: the following implementations were found:
              <std::string::String as std::convert::From<&str>>
              <std::string::String as std::convert::From<std::borrow::Cow<'a, str>>>
              <std::string::String as std::convert::From<std::boxed::Box<str>>>
    = note: required because of the requirements on the impl of `std::convert::Into<std::string::String>` for `&std::string::String`

error[E0277]: the trait bound `std::string::String: std::convert::From<&std::string::String>` is not satisfied
   --> src/main.rs:251:29
    |
251 |             content = loop_.into();
    |                             ^^^^ the trait `std::convert::From<&std::string::String>` is not implemented for `std::string::String`
    |
    = help: the following implementations were found:
              <std::string::String as std::convert::From<&str>>
              <std::string::String as std::convert::From<std::borrow::Cow<'a, str>>>
              <std::string::String as std::convert::From<std::boxed::Box<str>>>
    = note: required because of the requirements on the impl of `std::convert::Into<std::string::String>` for `&std::string::String`

[reitermarkus]

Merged in https://github.com/reitermarkus/cargo-eval/commit/265c63c01432303c9596e0e042b303cf5308d849.