examples: show how to build for Wasm

Tink requires cryptographic randomness, and this is provided by the
`getrandom` crate. By default, that crate does not include any support
for Wasm, but the `js` feature turns on support for randomness via WASI.

Modify one of the examples to demonstrate how to build for Wasm by
ensuring that `getrandom/js` is in the Cargo build graph.

Check this with a CI step.

Author: daviddrysdale

.github/workflows/ci.yml

@@ -165,6 +165,17 @@ jobs:
       - run: rustc --version
       - run: cargo doc --no-deps --document-private-items --all-features
 
+  wasm:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@629c2de402a417ea7690ca6ce3f33229e27606a5 # v2
+      - uses: actions-rs/toolchain@63eb9591781c46a70274cb3ebdf190fce92702e8 # v1
+        with:
+          profile: minimal
+          toolchain: 1.81.0
+          target: wasm32-unknown-unknown
+      - run: (cd examples/aead && cargo +1.81.0 build --target wasm32-unknown-unknown --release)
+
   udeps:
     runs-on: ubuntu-latest
     steps:

Cargo.lock

@@ -7,6 +7,9 @@ license = "Apache-2.0"
 publish = false
 
 [dependencies]
+# Force on the `js` feature of `getrandom` so a build of this example
+# for `wasm32-unknown-unknown` works.
+getrandom = { version = "^0.2.8", features = ["js"] }
 hex = "^0.4.3"
 tink-aead = "^0.3"
 tink-core = "^0.3"