Pull stream find

A pull-stream version of Array.prototype.find returning a promise.

``js
find(test)
`

`js
const {pull, values, map} = require('pull-stream')
const find = require('psp-find')

pull(
values([1, 2, 3, 4]),
find((x) => x > 2)
).then(console.log)
// 3

pull(
values([1, 2, 3]),
find((x) => x + 3 === 5)
).then(console.log)
// 2

pull(
values([1, 2, 3]),
find((x) => x > 4)
).then(console.log)
// undefined
``