Only enable instant/wasm-bindgen when building for wasm32 architecture

[gferon]

This is the mirror change of emilk/egui#2808 to fix compilation with bazel.

[daxpedda]

While we are at it, I would like to suggest removing it completely from native builds.
There are only two places that need a cfg guard anyway:

• winit/src/event.rs

  Line 37 in 77f8e51

  use instant::Instant;

• winit/src/event_loop.rs

  Line 14 in 77f8e51

  use instant::{Duration, Instant};

Examples can be kept by just adding it to the dev-dependencies ... only one example anyway.