feature: implement inner attributes capability

[Carter12s]

Attempts to solve #149

Not super familiar with macro stuff, so I maybe went overkill on the testing, but solution should be good.

[palfrey]

1.68.2.patch
Took a bit of fiddling, but here's a patch to make the ntest tests build with 1.68.2, as I think they're a good idea. I admit the MSRV does result in some trickiness, but doing things this way around makes sure it's compatible, even if we don't strictly need serial_test_test itself to be compatible!

[palfrey]

Couple of minor items, but overall very happy with this. Macros are a lot less scary than people think once you get your head around the core concepts :)

[Carter12s]

Adopted suggested error message change. Thanks for figuring out the lockfile changes to get ntest to build, I also like those tests and was annoyed when I couldn't figure out a working setup with 1.68.

Overall this MR was pretty easy to do, your existing code was very readable and a great help in figuring out the right way to add this! Thanks for being an awesome maintainer!

[Carter12s]

@palfrey Bump on this, I believe I've made the needed changes

[palfrey]

Apologies, life got in the way!