Macro py_run_impl 

#[doc(hidden)]
macro_rules! py_run_impl {
    ($py:expr, $($val:ident)+, $code:expr) => { ... };
    ($py:expr, *$dict:expr, $code:expr) => { ... };
}

Internal implementation of the py_run! macro.

FIXME: this currently unconditionally allocates a CString. We should consider making this not so:

• Maybe require users to pass &CStr / CString?
• Maybe adjust the unindent code to produce &Cstr / Cstring?