Restore compatibility with rustc older than 1.43

    error[E0599]: no associated item named `MAX` found for type `isize` in the current scope
       --> src/identifier.rs:106:37
        |
    106 |         debug_assert!(len <= isize::MAX as usize);
        |                                     ^^^ associated item not found in `isize`
        |
    help: you are looking for the module in `std`, not the primitive type
        |
    106 |         debug_assert!(len <= std::isize::MAX as usize);
        |                              ^^^^^^^^^^^^^^^

    error[E0599]: no associated item named `MAX` found for type `usize` in the current scope
       --> src/identifier.rs:132:44
        |
    132 |                     let max_alloc = usize::MAX / 2 - align;
        |                                            ^^^ associated item not found in `usize`
        |
    help: you are looking for the module in `std`, not the primitive type
        |
    132 |                     let max_alloc = std::usize::MAX / 2 - align;
        |                                     ^^^^^^^^^^^^^^^

src/identifier.rs

@@ -67,11 +67,13 @@
 // allows size_of::<Version>() == size_of::<Option<Version>>().
 
 use crate::alloc::alloc::{alloc, dealloc, handle_alloc_error, Layout};
+use core::isize;
 use core::mem;
 use core::num::{NonZeroU64, NonZeroUsize};
 use core::ptr::{self, NonNull};
 use core::slice;
 use core::str;
+use core::usize;
 
 const PTR_BYTES: usize = mem::size_of::<NonNull<u8>>();