Allow Ptr::offset() to receive any value that can cast to isize

The rationale behind this change is to avoid unnecessary casts that may
hurt readability, such as: `0_usize as isize`.
With this change, we are allowed to omit the cast, as it will be
performed inside the `offset` function.

This also has the advantage of panicking whenever we try to cast a usize
larger than `isize::MAX`.

Author: FranciscoTGouveia

libcc2rs/src/rc.rs

@@ -296,7 +296,11 @@ impl<T> Ptr<T> {
     }
 
     #[inline]
-    pub fn offset(&self, offset: isize) -> Self {
+    pub fn offset(&self, offset: impl TryInto<isize>) -> Self {
+        let offset = match offset.try_into() {
+            Ok(o) => o,
+            Err(_) => panic!("the offset must fit in a isize"),
+        };
         let step = self.elem_step();
         Self {
             kind: self.kind.clone(),