Use MAX associated constant

```
warning: use of constant `std::isize::MAX` that will be deprecated in a future Rust version: replaced by the `MAX` associated constant on this type
   --> src/utils.rs:100:68
    |
100 |         if !new_align.is_power_of_two() || new_size > core::isize::MAX as usize - (new_align - 1) {
    |                                                                    ^^^
    |
    = note: requested on the command line with `-W deprecated-in-future`

warning: use of associated function `core::num::<impl isize>::max_value` that will be deprecated in a future Rust version: replaced by the `MAX` associated constant on this type
   --> src/raw.rs:313:47
    |
313 | ...                   if state > isize::max_value() as usize {
    |                                         ^^^^^^^^^

warning: use of associated function `core::num::<impl isize>::max_value` that will be deprecated in a future Rust version: replaced by the `MAX` associated constant on this type
   --> src/raw.rs:343:27
    |
343 |         if state > isize::max_value() as usize {
    |
```

src/raw.rs

@@ -310,7 +310,7 @@ where
                         // If the task is not running, now is the time to schedule.
                         if state & RUNNING == 0 {
                             // If the reference count overflowed, abort.
-                            if state > isize::max_value() as usize {
+                            if state > isize::MAX as usize {
                                 abort();
                             }
 
@@ -340,7 +340,7 @@ where
         let state = (*raw.header).state.fetch_add(REFERENCE, Ordering::Relaxed);
 
         // If the reference count overflowed, abort.
-        if state > isize::max_value() as usize {
+        if state > isize::MAX as usize {
             abort();
         }
 

src/utils.rs

@@ -97,7 +97,7 @@ impl Layout {
         // - align is 0 (implied false by is_power_of_two())
         // - align is not a power of 2
         // - size rounded up to align overflows
-        if !new_align.is_power_of_two() || new_size > core::isize::MAX as usize - (new_align - 1) {
+        if !new_align.is_power_of_two() || new_size > isize::MAX as usize - (new_align - 1) {
             return None;
         }