Browser: Support crypto_kx_ functions

[jwerle]

Expected Behavior

crypto_kx_* API are callable functions.

Actual Behavior

crypto_kx_* functions are undefined.

Currently, as mentioned in #21, we do not support the Key Exchange [1] API.

Steps to Reproduce the Problem

1. Attempt to load this module in a browser context
2. Notice that crypto_kx_* APIs are not functions

Specifications

• Version: 0.6.0
• Platform: Browser
• Subsystem: kx.js

[1] - https://download.libsodium.org/doc/key_exchange/