Allow proto::extends default ctor to leave the proto_expr_ member uninitialized

[antoine.de-maricourt@…]

Proposal:

turn default ctor for proto::extends in proto/extends.hpp from:

extends(): proto_expr_(){}

to:

 extends() { }

Rational: It is up to the derived class to decide if expr can be left in undefined state (and to control it), and not to the extends base class.

[Eric Niebler]

After considering this, I've decided that there isn't sufficient justification for extends to leave its member uninitialized. I recommend BOOST_PROTO_EXTENDS for when you don't want to initialize the member.