Remove PhpParser dependency

[reimic]

The reimplemented Jason Mapper uses less dependencies than before and works well with PHP 7.0 and up. Yet, we can still reduce more.

One of the things to consider is the PhpParser. It looks as if we could drop in in favor of reimplementation or a smaller dependency. Let's look into that.

This package is 250kb zipped. It's fine for this PR but let's explore switching to something lightweight like token_get_all() before shipping v1 of this library. Would you please start an issue to track that?
Originally posted by @adamziel in WordPress/blueprints#69 (comment)

[reimic]

The reimplemented Json Mapper does not use the PhpParser. Other dependencies were ported in #77 and also do not require its presence.