Downgrade requirements for WASM dependencies

WebAssembly support was added in strawlab#38 targeting the current versions
versions of wasm-bindgen and js-sys at the time, which is
reasonable. Unfortunately the whoami crate, in trying to support
older Rust versions, has reduced the upper version of wasm-bindgen
that it supports, see ardaku/whoami#43.

This PR thus reduces the minimum version required for wasm-bindgen.
I used `cargo +nightly update -Z minimal-versions` with
`cargo build --target wasm32-unknown-unknown` to find the earliest
version that compiles. We could theoretically set up checks in
CI to verify this on an ongoing basis, but this might be overkill
for now.

Cargo.toml

@@ -25,8 +25,8 @@ core-foundation-sys = "0.8.3"
 winapi = { version = "0.3.9", features = ["activation", "combaseapi", "objbase", "roapi", "winerror", "winstring"] }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
-js-sys = "0.3.58"
-wasm-bindgen = "0.2.81"
+js-sys = "0.3.46"
+wasm-bindgen = "0.2.70"
 
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
-wasm-bindgen-test = "0.3.31"
+wasm-bindgen-test = "0.3"