Implement Debug for std::vec::IntoIter.

[frewsxcv]

Display all the remaining items of the iterator, similar to the Debug
implementation for core::slice::Iter:

rust/src/libcore/slice.rs

Lines 930 to 937 in f0bab98

	#[stable(feature = "core_impl_debug", since = "1.9.0")]
	impl<'a,T:'a + fmt::Debug> fmt::DebugforIter<'a,T>{
	fnfmt(&self,f:&mut fmt::Formatter) -> fmt::Result{
	f.debug_tuple("Iter")
	.field(&self.as_slice())
	.finish()
	}
	}

Using the as_slice method that was added in:

#35447

[alexcrichton]

@bors: r+ bc52bdc

Thanks!

[eddyb]

@bors rollup