[oden] The big lifetime removal

It turns out that rust can't really reason about the relationship
between the runtime lifetime and the context lifetime in a way that is
actually usable. This removes the lifetime stuff in favor of reference
counting the runtime itself, via a block that we embed in the
pointer. This, I think, it the least worst option here.

oden-js/src/runtime.rs

@@ -1,4 +1,9 @@
 use oden_js_sys as sys;
+use std::cell::RefCell;
+
+struct PrivateState {
+    refs: u64,
+}
 
 #[derive(Debug)]
 pub struct Runtime {
@@ -7,7 +12,23 @@ pub struct Runtime {
 
 impl Runtime {
     pub fn new() -> Runtime {
-        let rt = unsafe { sys::JS_NewRuntime() };
+        let state = Box::new(RefCell::new(PrivateState { refs: 1 }));
+        let rt = unsafe {
+            let rt = sys::JS_NewRuntime();
+            sys::JS_SetRuntimeOpaque(rt, Box::into_raw(state) as *mut _);
+            rt
+        };
+        Runtime { rt }
+    }
+
+    pub(crate) fn from_raw(rt: *mut sys::JSRuntime) -> Self {
+        let mut state = unsafe {
+            let ptr = sys::JS_GetRuntimeOpaque(rt) as *const RefCell<PrivateState>;
+            ptr.as_ref()
+                .expect("We already know this runtime is one of ours!")
+                .borrow_mut()
+        };
+        state.refs += 1;
         Runtime { rt }
     }
 
@@ -37,11 +58,36 @@ impl Runtime {
     }
 }
 
+impl Clone for Runtime {
+    fn clone(&self) -> Self {
+        Runtime::from_raw(self.rt)
+    }
+}
+
 impl Drop for Runtime {
     fn drop(&mut self) {
-        unsafe {
-            sys::JS_RunGC(self.rt);
-            sys::JS_FreeRuntime(self.rt);
+        let should_free = {
+            let mut state = unsafe {
+                let ptr = sys::JS_GetRuntimeOpaque(self.rt) as *const RefCell<PrivateState>;
+                ptr.as_ref()
+                    .expect("We already know this runtime is one of ours!")
+                    .borrow_mut()
+            };
+            state.refs -= 1;
+            state.refs == 0
+        };
+
+        if should_free {
+            unsafe {
+                let opaque = sys::JS_GetRuntimeOpaque(self.rt);
+                sys::JS_RunGC(self.rt);
+                sys::JS_FreeRuntime(self.rt);
+
+                if !opaque.is_null() {
+                    // Just let the system drop it here.
+                    let _ = Box::from_raw(opaque as *mut RefCell<PrivateState>);
+                }
+            }
         }
     }
 }