Update for Purescript 0.12

[mlang]

Summary:

• Move from purescript-eff to purescript-effect.
• Convert the gain example.
• Update dependencies.

[adkelley]

The work to migrate purescript-webaudio was completed shortly after the release of 0.12. You'll find it on pursuit: https://pursuit.purescript.org/packages/purescript-webaudio/0.1.2. If you're interested in contributing then please fork it: https://github.com/adkelley/purescript-webaudio