Enable building for Web Assembly target

The `rand` crate won't build properly for the
`wasm32-unknown-unknown` target unless we also depend
directly on the `getrandom` crate and enable its `js` feature.

References:
* rust-random/rand#886
* https://docs.rs/getrandom/0.2/getrandom/#webassembly-support

Cargo.toml

@@ -22,6 +22,9 @@ log = "0.4"
 rand = "0.8"
 serde = { version = "1.0", features = ["derive"] }
 
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+getrandom = { version = "0.2", features = ["js"] }
+
 [dev-dependencies]
 maplit = "1.0"
 simple_logger = "1.11"