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This commit moves all tests to being compiled with the 2018 edition, and
adds one standlone test which is compiled with the 2015 edition. One
`use` statement needed an update but otherwise everything looks like
it's good to go for 2018!

Closes #35

It creates a new extension-traits with can contain the implementation

* No longer necessary to `extern crate`
* Add `<'_>` where idiomatic

I was reading over the gnome-class macro crate recently and I was having
a difficult time getting up to speed on the error handling of the crate
as it was using a number of patterns I hadn't seen before. I was also
wondering how only one `Span` was necessary to create an error when
multi-token spans were needed sometimes, which led me in the end to
start a refactor here.

The first point of refactoring was to ensure that multi-token errors
were handled. Although the `syn` crate has a `Spanned` trait it doesn't
actually work on stable Rust, so the errors coming out of gnome-class
today weren't compatible with being spanned over multiple tokens. I
added a convenience API in https://github.com/dtolnay/syn/pull/538 and
have started using that here to construct syn `Error` instances.

Next I opted to lift similar error handling from the `failure` crate
here as well. This was primarily the `bail!` macro as well as the
`format_err!` macro, used to quickly return and create errors with
formatted messages.

Using this macro I was able to replace most of the support traits in
`src/errors.rs` with local error handling and/or various other idioms.
Afterwards the crate (to me at least) feels much more idiomatic in terms
of error handling where `?` is still maximally used, returning an error
is easy to do, and there aren't too many traits/methods to discover when
learning how to work with errors.

The library is building against rust 2018 but the test can't
compile succesfully against that.

A few changes included in this small refactoring:

* The return value is asserted to not be `G_TYPE_INVALID`
* Instead of `catch_unwind` a `Drop` implementation is used.
* When spinning, use `yield_now` to ensure that not too much work is
  wasted

Closes #39

Looks like we are hitting a compiler bug so we need to enable
`extern_crate_item_prelude`. Should be fixed by the time
Rust 2018 is released though.

[1] https://github.com/rust-lang/rust/issues/55599

Be sure to special case `Unit` return types to avoid generating these
unnecessary tokens!

Closes #41

Closes #43

Fixes federico/gnome-class#36

The syntax is

   impl MyClass {
        reserve_slots(10)
   }

Fixes federico/gnome-class#4

It works, but if we just do

  class Foo {
  }

  impl Foo {
      virtual fn blah(&self, ...) {}
  }

  class Bar: Foo {
  }

  // i.e. with no "impl Foo for Bar"

then this won't work:

  let bar = Bar::new();
  bar.blah(); // no method named `blah` found for type `Bar` in the current scope

We in fact have to specify

  impl Foo for Bar {}

Will fix this later.

It works!!!!

test.