internal/task: use asyncify on webassembly

This change implements a new "scheduler" for WebAssembly using binaryen's asyncify transform.
This is more reliable than the current "coroutines" transform, and works with non-Go code in the call stack.

runtime (js/wasm): handle scheduler nesting

If WASM calls into JS which calls back into WASM, it is possible for the scheduler to nest.
The event from the callback must be handled immediately, so the task cannot simply be deferred to the outer scheduler.
This creates a minimal scheduler loop which is used to handle such nesting.

.github/workflows/windows.yml

@@ -20,6 +20,10 @@ jobs:
         run: |
           choco install qemu --version=2020.06.12
           echo "C:\Program Files\QEMU" >> $GITHUB_PATH
+      - name: Install Ninja
+        shell: bash
+        run: |
+          choco install ninja
       - name: Checkout
         uses: actions/checkout@v2
         with:
@@ -50,8 +54,6 @@ jobs:
           # fetch LLVM source
           rm -rf llvm-project
           make llvm-source
-          # install dependencies
-          choco install ninja
           # build!
           make llvm-build
           # Remove unnecessary object files (to reduce cache size).
@@ -65,6 +67,15 @@ jobs:
       - name: Build wasi-libc
         if: steps.cache-wasi-libc.outputs.cache-hit != 'true'
         run: make wasi-libc
+      - name: Cache Binaryen
+        uses: actions/cache@v2
+        id: cache-binaryen
+        with:
+          key: binaryen-v1
+          path: build/binaryen
+      - name: Build Binaryen
+        if: steps.cache-binaryen.outputs.cache-hit != 'true'
+        run: make binaryen
       - name: Test TinyGo
         shell: bash
         run: make test