Serialize BigInt as an Object.

Not sure I want to keep this, though. It is necessary for IndexedDB, but
it suggests that IndexedDB ought to have its own peculiar flavor of
serialization in the `indexeddb` module if this is necessary.

See #22.

test/verbatim.t.js

@@ -1,4 +1,4 @@
-require('proof')(21, okay => {
+require('proof')(22, okay => {
     const Verbatim = require('..')
 
     function cycle (value) {
@@ -42,4 +42,11 @@ require('proof')(21, okay => {
     okay(buffer.toString(), 'a', 'buffer')
 
     okay(cycle({ a: null, b: null }), { a: null, b: null }, 'null should no created references')
+
+    // This is getting dubious. I'm not sure I want to support this sort of
+    // thing since it is nothing I'd ever want in my own software. I'm adding it
+    // for support of IndexedDB, but if that's the way it's going to be,
+    // preserving nonsense values like BigInt as Object, then I'll probably
+    // create a copy of Verbatim that is specific to IndexedDB.
+    okay(cycle(Object(1n)).valueOf(), 1n, 'bigint object')
 })

verbatim.js

@@ -15,6 +15,8 @@ exports.serialize = function (object) {
                     array.push(serialize(path.concat(i), value[i], buffers))
                 }
                 return array
+            } else if (value instanceof BigInt) {
+                return [ '_bigint_object', String(value.valueOf()) ]
             } else if (value instanceof Date) {
                 return [ '_date', value.getTime() ]
             } else if (Buffer.isBuffer(value)) {
@@ -78,6 +80,8 @@ exports.deserialize = function (buffers) {
                 return function () {} ()
             case '_bigint':
                 return BigInt(value[1])
+            case '_bigint_object':
+                return Object(BigInt(value[1]))
             }
         }
         if (value == null) {