Fix trait bounds for recent Rust nightly

[veeenu]

On recent Rust nightlies, this crate fails to compile with the following error:

error[E0059]: type parameter to bare `Fn` trait must be a tuple
   --> C:\Users\andrea\.cargo\git\checkouts\detour-rs-4ad9facba90b8dd7\b0e6366\src\detours\statik.rs:106:8
    |
106 |     D: Fn<T::Arguments, Output = T::Output> + Send + 'static,
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Tuple` is not implemented for `<T as Function>::Arguments`
    |
note: required by a bound in `Fn`
help: consider further restricting the associated type
    |
106 |     D: Fn<T::Arguments, Output = T::Output> + Send + 'static, <T as Function>::Arguments: Tuple
    |                                                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

error[E0059]: type parameter to bare `Fn` trait must be a tuple
   --> C:\Users\andrea\.cargo\git\checkouts\detour-rs-4ad9facba90b8dd7\b0e6366\src\detours\statik.rs:157:8
    |
157 |     C: Fn<T::Arguments, Output = T::Output> + Send + 'static,
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Tuple` is not implemented for `<T as Function>::Arguments`
    |
note: required by a bound in `Fn`
help: consider further restricting the associated type
    |
157 |     C: Fn<T::Arguments, Output = T::Output> + Send + 'static, <T as Function>::Arguments: Tuple
For more information about this error, try `rustc --explain E0059`.

This patch addresses this issue.

P.S. I also added a rust-toolchain.toml file as it looks like this crate fails to compile without nightly anyway. Is that desirable?