Improved support for nullables, tuples, shapes, and callables

• ScannedTypehint::getTypeName() will never include the ? token
• ScannedTypehint::isNullable() was previously returning true for any type containing a ? token; this is inaccurate for types such as tuple(?string, string). Fixed.
• ScannedTypehint::getTypeName() will now return 'tuple', 'shape', 'callable' if appropriate; the full declaration is still available via ScannedTypehint::getTypeText()
• callables and tuple type names and type text are no longer incorrectly prefixed with the current namespace
• added support for the future use namespace syntax in Hack