Update CodeMemory to be Send + Sync (#780)

* Update `CodeMemory` to be `Send + Sync`

This commit updates the `CodeMemory` type in wasmtime to be both `Send`
and `Sync` by updating the implementation of `Mmap` to not store raw
pointers. This avoids the need for an `unsafe impl` and leaves the
unsafety as it is currently.

* Run rustfmt

* Rename `offset` to `ptr`

crates/jit/src/code_memory.rs

@@ -14,6 +14,11 @@ pub struct CodeMemory {
     published: usize,
 }
 
+fn _assert() {
+    fn _assert_send_sync<T: Send + Sync>() {}
+    _assert_send_sync::<CodeMemory>();
+}
+
 impl CodeMemory {
     /// Create a new `CodeMemory` instance.
     pub fn new() -> Self {