Fix borrow scope for store in WrappedCallable impl for WasmtimeFn.

This PR fixes the borrow scope of store in the `WrappedCallable` impl of
`WasmTimeFn` such that it does not remain borrowed across the call to
`wasmtime_call_trampoline`. By limiting the scope of the borrow, the
implementation can be reentered if an exported function calls an imported
function, which in turn calls another exported function.

Fixes #365.

wasmtime-api/src/callable.rs

@@ -56,9 +56,6 @@ impl WrappedCallable for WasmtimeFn {
             _ => panic!("unexpected export type in Callable"),
         };
 
-        let mut store = self.store.borrow_mut();
-
-        let context = store.context();
         let value_size = mem::size_of::<u64>();
         let mut values_vec: Vec<u64> = vec![0; max(params.len(), results.len())];
 
@@ -78,7 +75,10 @@ impl WrappedCallable for WasmtimeFn {
         }
 
         // Get the trampoline to call for this function.
-        let exec_code_buf = context
+        let exec_code_buf = self
+            .store
+            .borrow_mut()
+            .context()
             .compiler()
             .get_published_trampoline(body, &signature, value_size)
             .map_err(|_| HostRef::new(Trap::fake()))?; //was ActionError::Setup)?;