v4.0.0rc1: new syntax, namespace support, modernization

This is a major new release; the most significant changes include:

• new xhp class foo syntax, replacing class :foo
• XHP classes can be declared in namespaces
• XHP classes can be used from namespaced code - the : character is treated as a namespace separator when instantiating classes
• XHP class names are no longer mangled; xhp class foo actually declares a runtime class called foo, not xhp_foo
• the children declaration is no longer supported; the approach introduced with 3.1 is now required
• the category declaration is no longer supported; interfaces are used instead
• all classes are now namespaced, including the HTML tags

This release currently requires:

• disable_xhp_element_mangling=true and enable_xhp_class_modifier=true in .hhconfig
• hhvm.hack.lang.enable_xhp_class_modifier=true and hhvm.hack.lang.disable_xhp_element_mangling=true in your HHVM configuration, including when generating the autoload map, running HHAST, etc.

Before the final release, expect:

• through migration instructions
• the defaults to change in HHVM so the above configuration is no longer required; this will effectively make XHP-Lib v4 the standard XHP-Lib for new HHVM releases