Update to latest rustfmt-nightly

To make future upgrades easier, we also make the following changes:

1. Use a rust-toolchain file to clearly specify the requirements of
our toolchain.
2. Refer to this toolchain in the CI configuration to avoid duplicating
the version requirement.
3. Remove `wasm` feature in favour of cfg-based dependency declarations.
4. Remove `patches` directory as they are now outdated with the recent
ap-crates releases.
5. Depend on rustfmt via git because the crates.io release is no longer
being updated. The Cargo.lock file tracks which specific Git revision
we depend on. Future updates therefore should only require a `cargo update`.

.github/workflows/ci.yml

@@ -17,23 +17,18 @@ jobs:
     env:
       CARGO_INCREMENTAL: 0
       RUST_BACKTRACE: full
-      CFG_RELEASE: 1.46.0-nightly
       CFG_RELEASE_CHANNEL: nightly
 
     steps:
     - uses: actions/checkout@v2
 
-    - name: Install and use nightly compiler
-      run: rustup default nightly-2020-06-14
+    - name: Install toolchain
+      run: rustup show # This will install the toolchain from the `rust-toolchain` file
 
-    - name: Install wasm32 target
-      if: matrix.config.kind == 'test_release'
-      run: rustup target add wasm32-unknown-unknown --toolchain nightly-2020-06-14
-
-    - name: Run setup script
+    - name: Extract Rust release version and set CFG_RELEASE env variable
       run: |
-        chmod +x setup.sh
-        ./setup.sh
+        # See https://sed.js.org/?gist=733942438b671befe20b6e7b5dfac57f for an explanation of the regex
+        echo "CFG_RELEASE=$(cargo --version | sed -rn 's/cargo\s(.*)\s\(.*\)/\1/p')" >> $GITHUB_ENV
 
     - name: Cache cargo
       uses: actions/cache@v2
@@ -49,7 +44,7 @@ jobs:
       run: cargo build --verbose
     - name: Build release
       if: matrix.config.kind == 'test_release'
-      run: cargo build --target wasm32-unknown-unknown --features "wasm" --release --verbose
+      run: cargo build --target wasm32-unknown-unknown --release --verbose
 
     - name: Test debug
       if: matrix.config.kind == 'test_debug'