Add mem and obj modules, move to nightly

* mem and obj modules are roughly divided between doing memory-specific
  things and object-specific things
* Box::new_uninit() is a nightly feature and it's quite useful, so I'm
  enabling that for now
* Check out src/obj/attrs.rs for possible undefined behavior in the
  Attrs::new() function. I'm sure it'll be just fine.

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

src/obj/ctx.rs

@@ -0,0 +1,26 @@
+use crate::mem::{gc::Gc, BasicGc};
+
+pub type DefaultGc = BasicGc;
+
+pub struct ObjCtx<G=DefaultGc>
+where
+    G: Gc,
+{
+    gc: G,
+}
+
+impl<G> ObjCtx<G>
+where
+    G: Gc,
+{
+    pub fn new(gc: G) -> Self {
+        ObjCtx { gc }
+    }
+    
+    pub fn gc(&self) -> &G {
+        &self.gc
+    }
+    pub fn gc_mut(&mut self) -> &mut G {
+        &mut self.gc
+    }
+}