Remove memoffset from dependencies

[Nugine]

I searched the usage of memoffset.
https://github.com/Manishearth/triomphe/search?q=offset_of
I find that there are only three offset_of! and the layout of data structures are stable. So it's easy to replace the macro with simple constants.

[Manishearth]

Rust layout is absolutely not stable

Though there may be stdlib builtins for offset now

[Nugine]

ArcInner and HeaderSlice have the attribute repr(C) so their layouts are stable.

triomphe/src/arc.rs

Lines 36 to 40 in b6a83f6

	#[repr(C)]
	pub(crate) struct ArcInner<T: ?Sized> {
	pub(crate) count: atomic::AtomicUsize,
	pub(crate) data: T,
	}

triomphe/src/header.rs

Lines 15 to 22 in df0434a

	#[repr(C)]
	pub struct HeaderSlice<H, T: ?Sized> {
	/// The fixed-sized data.
	pub header: H,
	/// The dynamically-sized data.
	pub slice: T,
	}

[Manishearth]

Ah. I would still like to not have to rely on hardcoded constants here

But I think stdlib has ptr_addr_of if you want to get rid of the dep

[Nugine]

ptr::addr_of{_mut} are better but they are added since Rust 1.51.0. Do you have a MSRV policy?

[Manishearth]

Not really no