Support for TypeAssert v3, disable attribute validation by default

• this release uses TypeAssert v3 instead of v2, allowing it to be used in new projects
• this release splits the API for enabling/disabling validation into separate options for child validation and attribute validation. :xhp::$ENABLE_VALIDATION no longer exists.
• attribute validation is off by default; we aim to remove it completely from a future release

Validation changes

Enabling attribute validation

:xhp::enableAttributeValidation();

Disabling child validation

:xhp::disableChildValidation();

Motivation

• This will allow removing the dependency on TypeAssert in the future
• The runtime validation isn't as useful now given that the typechecker validates attributes specified when instantiating XHP objects
• The runtime validation is bad for performance
• Keeping the runtime validation in sync with the typechecker has been a frequent source of issues

Caveat

As setAttribute() is not special-cased in the typechecker, so is now completely unchecked unless validation is enabled.