Revert "Support building with 1.57.0 compiler"

This reverts the changes from 66e3293f4d
to support the 1.57.0 compiler (leaving the rustdoc fixes).

Making the changes to support 1.57 felt disappointing to require for the
sake of Winit being able to better support Linux distro packaging. Even
with the changes we still wouldn't really support
1.57 without also upstreaming changes to `cargo ndk`.

The new plan is to bank on Winit bumping its own MSRV to at least
1.58 which seems more than reasonable considering that even 1.59
is already >6months old.

Ref: https://github.com/rust-windowing/winit/pull/2453

This updates ci.yml to now build with 1.60, assuming that the above
PR will be accepted.

.github/workflows/ci.yml

@@ -16,8 +16,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        # We need to support the same compiler versions as Winit
-        rust_version: [1.57.0, stable]
+        # We need to support the same MSRV as Winit which we are currently
+        # assuming will be bumped to 1.60.0 soon:
+        # https://github.com/rust-windowing/winit/pull/2453
+        rust_version: [1.60.0, stable]
     steps:
       - uses: actions/checkout@v2