Move wgpu re-export to root module

This seems to fix a `cargo doc` performance issue;
and it makes more sense anyways.

src/lib.rs

@@ -483,6 +483,9 @@ pub use iced_futures::stream;
 #[cfg(feature = "highlighter")]
 pub use iced_highlighter as highlighter;
 
+#[cfg(feature = "wgpu")]
+pub use iced_renderer::wgpu::wgpu;
+
 mod error;
 mod program;