Remove const_transmute feature dependency (#45)

Remove `const_transmute` feature flag

`const_transmute` has been stabilized on current nightly, see https://github.com/rust-lang/rust/pull/72920.
Document `const_fn_transmute` requirement when using `offset_of!` inside a `const fn`.
Add test for `offset_of!` inside a `const fn`.

README.md

@@ -68,7 +68,12 @@ features = ["unstable_const"]
 
 Your crate root: (`lib.rs`/`main.rs`)
 ```rust,ignore
-#![feature(ptr_offset_from, const_ptr_offset_from, const_transmute, const_raw_ptr_deref)]
+#![feature(ptr_offset_from, const_ptr_offset_from, const_raw_ptr_deref)]
+```
+
+Or, if you intend to use `offset_of!` inside a `const fn`:
+```rust,ignore
+#![feature(ptr_offset_from, const_fn, const_fn_transmute, const_ptr_offset_from, const_raw_ptr_deref)]
 ```
 
 and then:

src/lib.rs

@@ -61,8 +61,9 @@
     feature = "unstable_const",
     feature(
         ptr_offset_from,
+        const_fn,
+        const_fn_transmute,
         const_ptr_offset_from,
-        const_transmute,
         const_raw_ptr_deref,
     )
 )]

src/offset_of.rs

@@ -194,4 +194,21 @@ mod tests {
 
         assert_eq!([0; offset_of!(Foo, b)].len(), 4);
     }
+
+    #[cfg(feature = "unstable_const")]
+    #[test]
+    fn const_fn_offset() {
+        const fn test_fn() -> usize {
+            #[repr(C)]
+            struct Foo {
+                a: u32,
+                b: [u8; 2],
+                c: i64,
+            }
+
+            offset_of!(Foo, b)
+        }
+
+        assert_eq!([0; test_fn()].len(), 4);
+    }
 }