compiler: Use size_of from the prelude instead of imported

Use `std::mem::{size_of, size_of_val, align_of, align_of_val}` from the
prelude instead of importing or qualifying them.

These functions were added to all preludes in Rust 1.80.

compiler/rustc_serialize/src/leb128.rs

@@ -7,7 +7,7 @@ use crate::serialize::Decoder;
 /// Returns the length of the longest LEB128 encoding for `T`, assuming `T` is an integer type
 pub const fn max_leb128_len<T>() -> usize {
     // The longest LEB128 encoding for an integer uses 7 bits per byte.
-    (std::mem::size_of::<T>() * 8 + 6) / 7
+    (size_of::<T>() * 8 + 6) / 7
 }
 
 /// Returns the length of the longest LEB128 encoding of all supported integer types.