perf: small bl (#52)

This imports the `BufferList` class that does not extend duplex stream. This gets us one step closer to removing readable-stream from the bundle.

Author: Alan Shaw

src/chunker/fixed-size.js

@@ -1,6 +1,6 @@
 'use strict'
 
-const BufferList = require('bl')
+const BufferList = require('bl/BufferList')
 
 module.exports = async function * fixedSizeChunker (source, options) {
   let bl = new BufferList()

src/chunker/rabin.js

@@ -1,6 +1,6 @@
 'use strict'
 
-const BufferList = require('bl')
+const BufferList = require('bl/BufferList')
 const { create } = require('rabin-wasm')
 const errcode = require('err-code')