message: Add an UnpackValue for RefMut
  Summary: To mirror the one for ARef. Allows defining this arguments on mutable values more concisely and more safely.
diff: //! Parameter conversion utilities for `starlark_module` macros.
  -use crate::values::{Heap, Value};
  -use gazebo::cell::ARef;
  -use std::ops::Deref;
  +use crate::values::{ComplexValue, Heap, Value};
  +use gazebo::{any::AnyLifetime, cell::ARef};
  +use std::{cell::RefMut, ops::Deref};
  /// How to convert a [`Value`] to a Rust type. Required for all arguments in a [`#[starlark_module]`](macro@starlark_module) definition.
  pub trait UnpackValue<'v>: Sized {
  @@ -82,3 +82,9 @@ impl<'v, T: FromValue<'v>> UnpackValue<'v> for ARef<'v, T> {
  T::from_value(value)
  }
  }
  +
  +impl<'v, T: ComplexValue<'v> + AnyLifetime<'v>> UnpackValue<'v> for RefMut<'v, T> {
  + fn unpack_value(value: Value<'v>, heap: &'v Heap) -> Option<Self> {
  + value.downcast_mut(heap).ok()?
  + }
  +}
  