Remove memoffset dependency.

`std::mem::offset` has been availible on stable since 1.77.0.
`memoffset` uses this when compiled against a recent enough rustc.

Moving to the implementation in `std` lets us drop both `memoffset` and `autocfg` from our dependencies.

Cargo.toml

@@ -6,7 +6,7 @@ repository = "https://github.com/rust-analyzer/rowan"
 license = "MIT OR Apache-2.0"
 description = "Library for generic lossless syntax trees"
 edition = "2021"
-
+rust-version = "1.77.0"
 exclude = [".github/", "bors.toml", "rustfmt.toml"]
 
 [workspace]
@@ -18,7 +18,6 @@ hashbrown = { version = "0.14.3", features = [
     "inline-more",
 ], default-features = false }
 text-size = "1.1.0"
-memoffset = "0.9"
 countme = "3.0.0"
 
 serde = { version = "1.0.89", optional = true, default-features = false }

src/arc.rs

@@ -4,7 +4,7 @@ use std::{
     cmp::Ordering,
     hash::{Hash, Hasher},
     marker::PhantomData,
-    mem::{self, ManuallyDrop},
+    mem::{self, offset_of, ManuallyDrop},
     ops::Deref,
     ptr,
     sync::atomic::{
@@ -13,8 +13,6 @@ use std::{
     },
 };
 
-use memoffset::offset_of;
-
 /// A soft limit on the amount of references that may be made to an `Arc`.
 ///
 /// Going above this limit will abort your program (although not