Go 1.16 and darwin-arm64 (#381)

This commit switches to Go 1.16 and adds a release binary for darwin-arm64.

Fixes: #343

.github/workflows/gofmt.yml

@@ -14,10 +14,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - name: Set up Go 1.15
+    - name: Set up Go 1.16
       uses: actions/setup-go@v1
       with:
-        go-version: 1.15
+        go-version: 1.16
       id: go
 
     - name: Check out code into the Go module directory
@@ -26,9 +26,9 @@ jobs:
     - uses: actions/cache@v1
       with:
         path: ~/go/pkg/mod
-        key: ${{ runner.os }}-gofmt-${{ hashFiles('**/go.sum') }}
+        key: ${{ runner.os }}-gofmt1.16-${{ hashFiles('**/go.sum') }}
         restore-keys: |
-          ${{ runner.os }}-gofmt-
+          ${{ runner.os }}-gofmt1.16-
 
     - name: Install goimports
       run: |