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>

src/main.rs

@@ -1,3 +1,4 @@
+#![feature(unsize, coerce_unsized)]
 mod compile;
 mod obj;
 mod scope;