Remove dependency on memoffset for target_os = "windows"

[syberant]

Hi, thanks for firefox!
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 was stabilised in rustc 1.77.

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) 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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Your MSRV is unclear to me but this implies it's currently 1.90, maybe set it in Cargo.toml?
If that is correct this comment can also be followed up on.

[jschanck]

Thanks!

[micolous]

There are a couple of places in firefox-main where memoffset is used that probably aren't published on crates.io, so I opened a bug for removing those: https://bugzilla.mozilla.org/show_bug.cgi?id=2064304

With this dependency on memoffset 0.8 gone, we can also remove build/rust/memoffset, but we can handle that next time we sync this library across.

Thanks for flagging this!