Fix Rollup treeshaking

[drwpow]

Problem

When running Rollup on this library with the default behavior, it produces invalid JS:

// original (valid)
var wiggle = function wiggle(freq, amp) {
  // …
}.bind(this);

// rollup (invalid)
function wiggle(freq, amp) {
  // …
}.bind(this);

This is because it sees var wiggle is unused, which it is. It also occupies the same namespace as function wiggle.

Changes

This makes a simple change to the code. This makes it easier for Rollup (and other bundlers) to handle, with an invisible change:

- var wiggle = function wiggle(freq, amp) {
+ function wiggle(freq, amp) {
  // …
- }.bind(this);
+ }
+ wiggle.bind(this);

This is much easier for bundlers to prepare.

[drwpow]

Closing; this should be handled here: rollup/rollup#3950