Undefined behavior from MaybeUninit

[Jonathan Behrens]

Attempting to compile the crate with a recent nighly produces the following warning:

warning: the type `&mut reflect::Function` does not permit being left uninitialized
   --> src/reflect.rs:418:52
    |
418 |             let mut func: &mut Function = unsafe { std::mem::MaybeUninit::uninit().assume_init() };
    |                                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                                                    |
    |                                                    this code causes undefined behavior when executed
    |                                                    help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
    |
    = note: `#[warn(invalid_value)]` on by default
    = note: References must be non-null