Remove dependency on memoffset (#8304)

Hi, I may or may not have used your crate but I'd like to say a quick
thank you for it anyway!
I'm going down the list of reverse dependencies on `memoffset`.

This PR aims to remove the `memoffset` crate from your dependencies.

[`core::mem::offset_of`](https://doc.rust-lang.org/core/mem/macro.offset_of.html)
was stabilised in rustc 1.77 which I believe is at or below your MSRV.

The `memoffset` crate 0.9.1 says that

> If you're using a rustc version greater or equal to 1.77,
> this crate's offset_of!() macro simply forwards to
core::mem::offset_of!().

I consider it very unlikely (see
[here](https://github.com/rust-lang/rust/issues/111839)) for any usage
of the `offset_of!` macro to break but please check anyway.
I hope we can all enjoy the benefits of one less dependency :)

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* [x] I have followed the instructions in the PR template *except for
`./scripts/check.sh` which doesn't run in my environment* (I'm unwilling
to chase it down because I'm firing off a whole bunch of these PRs to
various repositories, sorry.)

`cargo clippy` gives 1 unrelated warning.

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

Cargo.toml

@@ -102,7 +102,6 @@ jiff = { version = "0.2.35", default-features = false }
 js-sys = "0.3.103"
 kittest = { version = "0.4.0" }
 log = { version = "0.4.33", features = ["std"] }
-memoffset = "0.9.1"
 mimalloc = "0.1.52"
 mime_guess2 = { version = "2.3", default-features = false }
 mint = "0.5.9"