Make sure frame examples compile for wasm (#5332)

* Make sure frame examples compile for wasm

This makes sure that `frame-example` and `frame-example-offchain-worker`
compile for wasm.

This also fixes compilation for these crates. The offchain worker
example doesn't use serde-json anymore as that is too heavy and breaks
`no_std` compilation.

* Apply suggestions from code review

Co-Authored-By: Nikolay Volf <nikvolf@gmail.com>

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>

substrate/.gitlab-ci.yml

@@ -214,6 +214,24 @@ test-frame-staking:
     - WASM_BUILD_NO_COLOR=1 time cargo test --release --verbose --no-default-features --features std
     - sccache -s
 
+test-frame-examples-compile-to-wasm:
+  stage:                           test
+  <<:                              *docker-env
+  variables:
+    # Enable debug assertions since we are running optimized builds for testing
+    # but still want to have debug assertions.
+    RUSTFLAGS: -Cdebug-assertions=y
+    RUST_BACKTRACE: 1
+  except:
+    variables:
+      - $DEPLOY_TAG
+  script:
+    - cd frame/example-offchain-worker/
+    - cargo +nightly build --target=wasm32-unknown-unknown --no-default-features
+    - cd ../example
+    - cargo +nightly build --target=wasm32-unknown-unknown --no-default-features
+    - sccache -s
+
 test-wasmtime:
   stage:                           test
   <<:                              *docker-env