Light-Client API for JavaScript

At present this just exposes the [Tendermint Light Client]'s verification logic via WASM. This allows simple access to verification from JavaScript:

```javascript import * as LightClient from 'tendermint-light-client-js';

// Verify an untrusted block against a trusted one, given the specified options // and current date/time. let verdict = LightClient.verify(untrusted, trusted, options, now); ```

For an example of how to use this, please see the [verifier-web example].