check wasm-bindgen against the version Cargo.lock pins

The generator refuses a module built by a different version, and the
error it gives names two versions without saying which is which. Reading
the wanted one out of the lock file also fixes the install hint, which
had been printing the requirement from Cargo.toml: "0.2", which is not
a version anything can be installed at.

npm/build.sh

@@ -17,9 +17,19 @@ version="$(sed -n 's/^version = "\(.*\)"/\1/p' Cargo.toml | head -1)"
 bindgen="${WASM_BINDGEN:-wasm-bindgen}"
 dist="npm/dist"
 
+# The generator has to be the exact version the crate was built against, or it
+# refuses the module. Cargo.lock is where that version actually is.
+want="$(awk '/^name = "wasm-bindgen"$/{getline; gsub(/[",]/,""); print $3; exit}' Cargo.lock)"
 command -v "$bindgen" >/dev/null || {
-  echo "wasm-bindgen is not on PATH. cargo install wasm-bindgen-cli --version \
-$(sed -n 's/^wasm-bindgen = { version = "\(.*\)".*/\1/p' Cargo.toml)" >&2
+  echo "wasm-bindgen is not on PATH." >&2
+  echo "  cargo install wasm-bindgen-cli --version $want" >&2
+  echo "or take the prebuilt one from" >&2
+  echo "  https://github.com/rustwasm/wasm-bindgen/releases/tag/$want" >&2
+  exit 1
+}
+have="$("$bindgen" --version | awk '{print $2}')"
+[ "$have" = "$want" ] || {
+  echo "wasm-bindgen $have, but Cargo.lock pins $want" >&2
   exit 1
 }