ci: Run target-agnostic build/test steps only once (#132)

* ci: Run target-agnostic build/test steps only once

A lot of these steps - except apk building - are not target-specific (do
not have a `--target` option; `target:` on `actions-rs/toolchain` only
specifies what targets to install). This is a waste of time despite all
duplication running in parallel, and could be used to run a
target-specific apk build in parallel with host-side tests instead.

It should be fine to perform just the apk build step on Windows, and
leave the rest (tests, documentation) to Linux runners only - most of
those steps were not ran on Windows anyway.

This takes CI time down from 6m30s to just under 5m, with around 3m
spent in apk building (in parallel), 3m40s spent testing all cates and
docs on the host, and under 5m for apk building on Windows
(defining the baseline for total build completion time).

* ci: Build-test docs separately

This build takes about a minute and seems to recheck all crates anyway,
and is hence better ran in parallel rather than synchronously with the
tests.

.github/workflows/rust.yml

@@ -34,6 +34,7 @@ jobs:
             rust-target: 'aarch64-linux-android'
 
     runs-on: ${{ matrix.os }}
+    name: Build apk
 
     steps:
     - uses: actions/checkout@v2
@@ -45,47 +46,80 @@ jobs:
         target: ${{ matrix.rust-target }}
         override: true
 
-    - if: runner.os != 'Windows'
-      name: Test ndk-sys
+    - name: Install cargo-apk
+      run:
+        cargo install --path cargo-apk
+
+    - name: Cargo check for target ${{ matrix.rust-target }}
+      run: cargo check -p ndk --target ${{ matrix.rust-target }} --all-features
+
+    - name: Cargo apk build for target ${{ matrix.rust-target }}
+      run: cargo apk build -p ndk-examples --target ${{ matrix.rust-target }} --examples
+
+  build-host:
+    strategy:
+      fail-fast: false
+      matrix:
+        os:  [ubuntu-latest]
+        rust-channel: ['stable', 'nightly']
+
+    runs-on: ${{ matrix.os }}
+    name: Host-side tests
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Installing Rust ${{ matrix.rust-channel }}
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: ${{ matrix.rust-channel }}
+        override: true
+
+    - name: Test ndk-sys
       run:
         cargo test -p ndk-sys --all-features
 
-    - if: runner.os != 'Windows'
-      name: Test ndk
+    - name: Test ndk
       run:
         cargo test -p ndk --all-features
 
     - name: Test ndk-build
       run:
         cargo test -p ndk-build --all-features
 
-    - if: runner.os != 'Windows'
-      name: Test ndk-glue
+    - name: Test ndk-glue
       run:
         cargo test -p ndk-glue --all-features
 
-    - if: runner.os != 'Windows'
-      name: Test ndk-macro
+    - name: Test ndk-macro
       run:
         cargo test -p ndk-macro --all-features
 
     - name: Test cargo-apk
       run:
         cargo test -p cargo-apk --all-features
 
-    - if: runner.os != 'Windows'
-      name: Document all crates
+  docs:
+    strategy:
+      fail-fast: false
+      matrix:
+        os:  [ubuntu-latest]
+        rust-channel: ['stable', 'nightly']
+
+    runs-on: ${{ matrix.os }}
+    name: Build-test docs
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Installing Rust ${{ matrix.rust-channel }}
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: ${{ matrix.rust-channel }}
+        override: true
+
+    - name: Document all crates
       env:
         RUSTDOCFLAGS: -Dwarnings
       run:
         cargo doc --all --all-features
-
-    - name: Install cargo-apk
-      run:
-        cargo install --path cargo-apk
-
-    - name: Cargo check for target ${{ matrix.rust-target }}
-      run: cargo check -p ndk --target ${{ matrix.rust-target }} --all-features
-
-    - name: Cargo apk build for target ${{ matrix.rust-target }}
-      run: cargo apk build -p ndk-examples --target ${{ matrix.rust-target }} --examples