Ads BigInt support#48

Beside adding BigInt support, this PR also ads node 12 to .travis.yml and updates standard to `^14.0.0` Not really sure about the new method names `jsonSafe` and `deterministicJsonSafe`. Resolves: #41 Had an earlier version which replaced all `JSON.stringify` calls, with a `bigintSafeJSONStringify` method (see below). But this had huge impacts on the performance (only about a third ops/sec than before). Therefore i decided to implement the toString conversion for BigInts directly in the former `decirc` method, hence the rename to `jsonSafe` as it does more then decirculate now. Performance is now about the same (sometimes faster, sometimes slower) as in master. ```js function bigintSafeJSONStringify (obj, replacer, spacer) { return JSON.stringify(obj, function (key, value) { replacer = replacer !== undefined ? replacer : function (k, v) { return v } return replacer(key, typeof value === 'bigint' ? value.toString() : value) }, spacer) } ``` Also i moved quite a bit of code from the former `decirc` method to a new `replaceProperty` method to avoid code duplication.