fix(Buffer): import buffer for binary, decimal128, and fnv1a

Fixes NODE-1891

Author: daprahamian

lib/binary.js

@@ -1,5 +1,7 @@
 'use strict';
 
+const Buffer = require('buffer').Buffer;
+
 /**
  * A class representation of the BSON Binary type.
  */

lib/decimal128.js

@@ -1,6 +1,7 @@
 'use strict';
 
 let Long = require('./long');
+const Buffer = require('buffer').Buffer;
 
 const PARSE_STRING_REGEXP = /^(\+|-)?(\d+|(\d*\.\d*))?(E|e)?([-+])?(\d+)?$/;
 const PARSE_INF_REGEXP = /^(\+|-)?(Infinity|inf)$/i;

lib/fnv1a.js

@@ -1,5 +1,6 @@
 'use strict';
 
+const Buffer = require('buffer').Buffer;
 const Long = require('./long');
 
 const MASK_8 = 0xff;