Expand wrap_pyfunction!() to a closure.

The change to wrap_pyfunction!() in PyO3#1143 makes it impossible to
implement `context.add_wrapped(wrap_pyfunction!(something))` in
`inline-python`.
`context` does not carry the GIL lifetime, which causes type deduction
trouble now that `wrap_pyfunction` results in a generic function.

```
error[E0308]: mismatched types
  --> examples/rust-fn.rs:12:4
   |
12 |     c.add_wrapped(wrap_pyfunction!(rust_print));
   |       ^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected enum `Result<&pyo3::types::PyCFunction, _>`
              found enum `Result<&pyo3::types::PyCFunction, _>`
```

By re-wrapping the function as a closure, we trigger 'closure signature
deduction' when passing `wrap_pyfunction!()` as an argument to a
function: Rustc will deduce the signature of the closure from the
function that closure is passed to. This way, the generic arguments can
be deduced in more contexts, fixing the problem.

src/lib.rs

@@ -221,7 +221,7 @@ pub mod proc_macro {
 #[macro_export]
 macro_rules! wrap_pyfunction {
     ($function_name: ident) => {{
-        &pyo3::paste::expr! { [<__pyo3_get_function_ $function_name>] }
+        &|py| pyo3::paste::expr! { [<__pyo3_get_function_ $function_name>] }(py)
     }};
 
     ($function_name: ident, $arg: expr) => {