Add syntax extension fourcc!()

I was looking into #9303 and was curious if this would still be valuable. @kballard had already done 99% of the work, so I brought the branch up to date and added a feature gate. Any feedback would be appreciated; I wasn't sure if this should be set up as a syntax extension with `#[macro_registrar]`, and if so, where it should be located.

Original PR is here: #9255