Rust features wanted for jrsonnet

[CertainLach]

To remove as_any[_mut] methods

Some traits in jrsonnet have as_any/as_any_mut methods
Those methods might be removed by making Any a supertrait of those, but it will be only possible with

• Tracking issue for dyn upcasting coercion rust-lang/rust#65991

Better GC

Many good Rust GC designs with explicit root tracking (Like https://github.com/withoutboats/shifgrethor) require method receivers to be typed as in fn method(self: Gc<Self>), this requires

• Tracking issue for RFC 3519: arbitrary_self_types rust-lang/rust#44874

(This is also possible to implement with rustc_driver, which implements required linting, as in servo... But this is uglier, and requires using separate compiler)

Until then, current Gc is a reference counting with cycle collection instead, as we can't reliably track roots.
rust-gc, which was used previously, spends too much time tracking roots, so its approach is not practical.