reflect: avoid deadlock in GenericTypeCell

[soqb]

Objective

• There was a deadlock discovered in the implementation of bevy_reflect::utility::GenericTypeCell, when called on a recursive type, e.g. Vec<Vec<VariableCurve>>

Solution

• Drop the lock before calling the initialisation function, and then pick it up again afterwards.

Additional Context

• Discussed on Discord

[MrGVSV]

LGTM!

---

One issue this brings up (and that this PR does not need to be the one to fix) is that we apparently don't support self-referential types. The following won't compile:

#[derive(Reflect, FromReflect)]
struct Foo {
  foo: Vec<Foo>
}

Errors

error[E0275]: overflow evaluating the requirement `tests::recursive_typed_storage_does_not_hang::Foo: type_path::TypePath`
    --> crates/bevy_reflect/src/lib.rs:1856:18
     |
1856 |         #[derive(Reflect, FromReflect)]
     |                  ^^^^^^^
     |
note: required for `Vec<tests::recursive_typed_storage_does_not_hang::Foo>` to implement `reflect::Reflect`
    --> crates/bevy_reflect/src/impls/std.rs:326:41
     |
326  |           impl<T: FromReflect + TypePath> Reflect for $ty {
     |                                 --------  ^^^^^^^     ^^^
     |                                 |
     |                                 unsatisfied trait bound introduced here
...

error[E0275]: overflow evaluating the requirement `tests::recursive_typed_storage_does_not_hang::Foo: from_reflect::FromReflect`
    --> crates/bevy_reflect/src/lib.rs:1856:27
     |
1856 |         #[derive(Reflect, FromReflect)]
     |                           ^^^^^^^^^^^
     |
note: required for `Vec<tests::recursive_typed_storage_does_not_hang::Foo>` to implement `from_reflect::FromReflect`
    --> crates/bevy_reflect/src/impls/std.rs:410:41
     |
410  |           impl<T: FromReflect + TypePath> FromReflect for $ty {
     |                   -----------             ^^^^^^^^^^^     ^^^
     |                   |
     |                   unsatisfied trait bound introduced here
...