[fix] Use simple mixin algorithm
[fix] Use simple mixin algorithm

`utile.mixin` uses `__lookupGetter__`, which, when invoked on
`process.env` throws this error:

    > process.env.__lookupGetter__('MANPATH')
    TypeError: Property '__lookupGetter__' of object #<error> is not a function
        at repl:1:14
        at REPLServer.self.eval (repl.js:111:21)
        at Interface.<anonymous> (repl.js:250:12)
        at Interface.EventEmitter.emit (events.js:88:17)
        at Interface._onLine (readline.js:199:10)
        at Interface._line (readline.js:517:8)
        at Interface._ttyWrite (readline.js:735:14)
        at ReadStream.onkeypress (readline.js:98:10)
        at ReadStream.EventEmitter.emit (events.js:115:20)
        at emitKey (readline.js:1057:12)

opsmezzo/quill