Make module information lookup from runtime safe.

This commit uses a two-phase lookup of stack map information from modules
rather than giving back raw pointers to stack maps.

First the runtime looks up information about a module from a pc value, which
returns an `Arc` it keeps a reference on while completing the stack map lookup.

Second it then queries the module information for the stack map from a pc
value, getting a reference to the stack map (which is now safe because of the
`Arc` held by the runtime).

crates/wasmtime/src/trampoline/func.rs

@@ -287,7 +287,7 @@ pub fn create_function(
                 host_state: Box::new(trampoline_state),
                 interrupts: std::ptr::null(),
                 externref_activations_table: std::ptr::null_mut(),
-                stack_map_lookup: None,
+                module_info_lookup: None,
             })?,
             trampoline,
         ))
@@ -319,7 +319,7 @@ pub unsafe fn create_raw_function(
             host_state,
             interrupts: std::ptr::null(),
             externref_activations_table: std::ptr::null_mut(),
-            stack_map_lookup: None,
+            module_info_lookup: None,
         })?,
     )
 }