Resolve ambiguous core import on rustc 1.64 through 1.71

In 1.72+, this is fixed by rust-lang/rust#112086.

    error[E0659]: `core` is ambiguous
       --> serde/src/lib.rs:227:13
        |
    227 |     pub use core::ffi::CStr;
        |             ^^^^ ambiguous name
        |
        = note: ambiguous because of multiple potential import sources
        = note: `core` could refer to a built-in crate
        = help: use `::core` to refer to this crate unambiguously
    note: `core` could also refer to the module defined here
       --> serde/src/lib.rs:166:5
        |
    166 | /     mod core {
    167 | |         #[cfg(not(feature = "std"))]
    168 | |         pub use core::*;
    169 | |         #[cfg(feature = "std")]
    170 | |         pub use std::*;
    171 | |     }
        | |_____^
        = help: use `self::core` to refer to this module unambiguously

serde/src/lib.rs

@@ -224,7 +224,7 @@ mod lib {
     pub use std::collections::{BTreeMap, BTreeSet, BinaryHeap, LinkedList, VecDeque};
 
     #[cfg(all(not(no_core_cstr), not(feature = "std")))]
-    pub use core::ffi::CStr;
+    pub use self::core::ffi::CStr;
     #[cfg(feature = "std")]
     pub use std::ffi::CStr;