crypto: use packages directly

src/crypto.js

@@ -1,4 +1,4 @@
-var crypto = require('crypto')
+var createHash = require('create-hash')
 
 function hash160 (buffer) {
   return ripemd160(sha256(buffer))
@@ -9,15 +9,15 @@ function hash256 (buffer) {
 }
 
 function ripemd160 (buffer) {
-  return crypto.createHash('rmd160').update(buffer).digest()
+  return createHash('rmd160').update(buffer).digest()
 }
 
 function sha1 (buffer) {
-  return crypto.createHash('sha1').update(buffer).digest()
+  return createHash('sha1').update(buffer).digest()
 }
 
 function sha256 (buffer) {
-  return crypto.createHash('sha256').update(buffer).digest()
+  return createHash('sha256').update(buffer).digest()
 }
 
 module.exports = {