Add basic hack language support

[thekid]

See xp-framework/rfc#308

Scope

This pull request supersedes #57 and implements the following Hack features in XP:

• Running hack code alongside PHP - seamless
• The short hack lambdas inside regular code - seamless
• Hack parameter and return types, including void, this, noreturn, the num and arraykey unions, function types and typed arrays array<T> and maps array<?, T> - via changes to type system
• Hack enums to be recognized as enum types correctly - by checking for HH\BuiltinEnum

Hack generics and attributes are not supported. See this comment on #122: We only cover the very basics to make reflection work consistently with PHP and Hack. Other features need to be implemented (or already are) in libraries such as https://github.com/xp-forge/mirrors!

Example

Test.class.php:

<?hh

use util\cmd\Console;

class Test {

  public static function main(array<string> $args): int {
    Console::writeLine('Hello World');
    return 0;
  }
}

Load and run:

vagrant@vagrant-ubuntu-vivid-64:/devel/xp/core$ XP_RT=hhvm xp Test
Hello World

Reflect:

vagrant@vagrant-ubuntu-vivid-64:/devel/xp/core$ XP_RT=hhvm xp -w \
  '\lang\XPClass::forName("Test")->getMethods()'
[public static int main(array<string> $args)]
vagrant@vagrant-ubuntu-vivid-64:/devel/xp/core$ XP_RT=hhvm xp -w \
  '\lang\XPClass::forName("Test")->getMethods ()[0]->getParameters()[0]->getType()'
lang.ArrayType<string[]>