Sync with V8 upstream and new features

I'm excited to announce that starting from this release documentation is available at https://php-v8.readthedocs.io.

Read The Docs is awesome place to host documentation and Sphinx would help to create decent documentation. Contributors welcomed!

This is the last version that supports PHP 7.1. For all next versions PHP >= 7.2 required.

BC-breaking changes:

• Upgrade v8 version to 6.6.313
• Remove deprecated and non-working ScriptCompiler cache options
• Rework PromiseObject
• Add ValueObject::isBigInt64Array() and ValueObject::isBigUint64Array methods

Non-breaking changes:

• Add number of native and detached contexts to HeapStatistics
• Add support to produce code cache
• Add ScriptCompiler::OPTION_EAGER_COMPILE option
• Add support for integer->strings for ObjectValue::GetOwnPropertyNames and `ObjectValue::GetPropertyNames
• Add {Template,ObjectValue}:setLazyDataProperty() method