derive: make error messages slightly more readable, add manually-driven test

[BenWiederhake]

Closes a bit of #129.

Using a fully-fledged compile-error testsuite is a bit overkill, but we still want to make sure that the derive crate generates reasonable error messages. That's what this "example" is for. In the following, there are blocks of lines, one marked as POSITIVE and multiple lines marked as NEGATIVE. The committed version of this file should only contain POSITIVE. In order to run a test, comment out the POSITIVE line, and use a NEGATIVE line instead, and manually check whether you see a reasonable error message – ideally the error message indicated by the comment. One way to do this is:

$ cargo build --example test_compile_errors_manually

Example output with the type that I did in #129:

$ cargo build --example test_compile_errors_manually
   Compiling uutils-args v0.1.0 (/home/user/workspace/uutils-args)
error: proc-macro derive panicked
  --> examples/test_compile_errors_manually.rs:24:10
   |
24 | #[derive(Arguments)]
   |          ^^^^^^^^^
   |
   = help: message: expected '=' after '[' in flag pattern

It's not a particularly nice error message, but it's more readable than the current called `Option::unwrap()` on a `None` value.

[BenWiederhake]

Changes since last push: cargo fmt

[tertsdiepraam]

I think this is a clear improvement, but maybe not enough to close the issue for two reasons.

The first is that I think a proper compile-test framework sounds pretty good for this crate, because it is so focused on derive-based functionality.

The second is that we would ideally use the spans from syn to generate error messages, so that the errors don't just get reported on the #[derive(Arguments)] but on the parts that are failing.

[BenWiederhake]

Ah, well, then I'll add compile-fail-tests in the next PR :)

[tertsdiepraam]

I unlinked the issue, but this is of course merge-worthy!