fix: make rabin an optional dependency

There are no pre-built binaries currently provided so it does not work
on systems that do not have suitable build tools installed, eg.
Windows out of the box.

package.json

@@ -72,10 +72,12 @@
     "pull-through": "^1.0.18",
     "pull-traverse": "^1.0.3",
     "pull-write": "^1.1.4",
-    "rabin": "^1.6.0",
     "sparse-array": "^1.3.1",
     "stream-to-pull-stream": "^1.7.2"
   },
+  "optionalDependencies": {
+    "rabin": "^1.6.0"
+  },
   "contributors": [
     "Alan Shaw <alan@tableflip.io>",
     "Arpit Agarwal <atvanguard@users.noreply.github.com>",

src/builder/builder.js

@@ -105,13 +105,20 @@ module.exports = function builder (createChunker, ipld, createReducer, _options)
     }
 
     const reducer = createReducer(reduce(file, ipld, options), options)
+    let chunker
+
+    try {
+      chunker = createChunker(options.chunkerOptions)
+    } catch (error) {
+      return callback(error)
+    }
 
     let previous
     let count = 0
 
     pull(
       file.content,
-      createChunker(options.chunkerOptions),
+      chunker,
       pull.map(chunk => {
         if (options.progress && typeof options.progress === 'function') {
           options.progress(chunk.byteLength)

src/chunker/rabin.js

@@ -1,9 +1,20 @@
 'use strict'
 
-const createRabin = require('rabin')
 const toPull = require('stream-to-pull-stream')
 
+let createRabin
+
 module.exports = (options) => {
+  if (!createRabin) {
+    try {
+      createRabin = require('rabin')
+    } catch (error) {
+      error.message = `Rabin chunker not supported, it may have failed to install - ${error.message}`
+
+      throw error
+    }
+  }
+
   let min, max, avg
   if (options.minChunkSize && options.maxChunkSize && options.avgChunkSize) {
     avg = options.avgChunkSize