They will be gated behind the "js" feature, as we can now do detect,
at compile-time, which implementation (wasm-bindgen vs stdweb) we
should use.

The "js" implementation takes precedence over the "custom"
implementation. This prevents issues that arise from the buggy way
Cargo handles features across multiple targets.

Signed-off-by: Joe Richey <joerichey@google.com>