allow calling assert_type{,_eq}! at top-level

[WaffleLapkin]

This PR allows using assert_type_eq! and assert_type! macros
at top-level.

This is done by using const _ instead of let _. consts could not be
uninitialized, so PhantomData is also used.

Note:
This requires rustc version 1.37.0 or higher (the version in which underscore_const_names was stabilized).
If such a bump of MSRV is not acceptable either of 2 things can be done:

• simply close this pr*
• update this pr, so in the macros, there will be 2 branches - one with const, and one without**

Personally, I would prefer the second one, but both are fine.

*anyway on 1.37+ you can do const _: () = { assert_type_eq!(A, B) };
**:

($a:ty, $b:ty) => {
    let _: <$a as $crate::Same<$b>>::Output;
};
(const $a:ty, $b:ty) => {
    const _: core::marker::PhantomData<<$a as $crate::Same<$b>>::Output> = 
core::marker::PhantomData;
};

[paholg]

Sorry for my delay; this looks good to me.

The last time I (accidentally) bumped the MSRV, it broke a dependent's MSRV build, and I reverted it. But it looks like they're on 1.41 now, and 1.37 seems plenty old enough.

There's a merge conflict because I changed build systems, but I can handle that.