Minor changes to ObjectReference based on the review

Author: qinsoon

src/util/address.rs

@@ -214,7 +214,7 @@ impl Address {
 
     /// Apply an signed offset to the address.
     pub const fn offset(self, offset: isize) -> Address {
-        Address((self.0 as isize).wrapping_add(offset) as usize)
+        Address(self.0.wrapping_add_signed(offset))
     }
 
     /// Bitwise 'and' with a mask.

src/vm/object_model.rs

@@ -433,7 +433,7 @@ pub trait ObjectModel<VM: VMBinding> {
     const VM_WORST_CASE_COPY_EXPANSION: f64 = 1.5;
 
     /// If this is true, the binding guarantees that the object reference's raw address,
-    /// the in-object address, and the object start is always the same address. To be precise,
+    /// the in-object address, and the object start are always the same address. To be precise,
     /// 1. an object reference's raw address is always equal to the return value of the `ref_to_object_start` method,
     /// 2. `IN_OBJECT_ADDRESS_OFFSET` is 0.
     ///