Add "nightly" feature to gc library, add unsize+coerce_unsized features

We're nightly, babey!!!

This is so we can coerce Gc<T> pointers to Gc<dyn Obj>

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

Cargo.toml

@@ -9,4 +9,4 @@ edition = "2021"
 thiserror = "1.0"
 structopt = "0.3"
 regex = "1.5"
-gc = { version = "0.4", features = ["derive"] }
+gc = { version = "0.4", features = ["derive", "nightly"] }