Fluid Variable enhancements

[Mark S. Miller <mark>]

I've fixed the fluid variable package in a mostly upwards compatable
fashion.  The changes are:

/*=======================================================================
  |
  |  DEFINE_FLUID(varName) -- DEFINE_FLUID is to DESIGN_FLUID
  |  				as DEFINE_CLASS is to CLASS
  |				except that type isn't repeated
  |	Invokes 0-parameter constructor to initialize the variable's
  |      storage.  Initialization happens at the time the fluid space
  |      is created--Fluid initialization time.  
  |
  =======================================================================*/

Programs which using DEFINE_FLUID on objects (as opposed to primitive
data types) were virtually guarateed of having undefined run-time
behavior (since the variable would never have been constructed in the
storage it was considered to have occupied).  Now the object will get
properly initialized.  The one non-upwards compatability is if you are
doing a DEFINE_FLUID on an object which has no 0-parameter
constructors.  Now you will get a compile time error (instead of
inexplicable run-time behavior).  On the other hand, if you want to
parameterize the initialization:

/*=======================================================================
  |
  |  BUILD_FLUID(varName,initExpr) -- BUILD_FLUID is like DEFINE_FLUID
  |	except that the storage is initialized by using the object's
  |	1-parameter constructor passing (initExpr) as the one argument.
  |	(initExpr) is also evaluated at fluid initialization time.
  |	Given the constraints of fluid initialization time, one should
  |	typically initialize fluids to simple constants (like IntegerVar0 
  |	or NULL).
  |
  =======================================================================*/

What's this new "Fluid Initialization Time" you ask?

/********************************************************************

  		   Fluid Initialization Time

Fluid initialization time is when the space for fluids storage gets
allocated, and when all the fluids which live in that space get
initialized.  There is a global fluids space, and a per-session
fluids space.  (The global fluids space can be considered to be
the space for the "main-session".)  Fluid initialization time
must happen after static initialization time, but may happen
before, during, or after dynamic initialization time.  The global 
space gets initialized on first use of a fluid, so dynamic initialers
can safely use fluids, but fluid initializers can only safely assume
static initialization.  Fluid initialization for each session happens
as part of the act of creating the session.

********************************************************************/

And finally, something I believe is already defined by some system
header file.  I couldn't find it so I redid it myself (I needed it for
the above):


/*=======================================================================
  |
  |  Invoked as "new (p) Foo (...)" in order to get the storage pointed
  |   to by p to be initialized to contain a Foo.  Taken verbatim from
  |   page 31 of "The Evolution of C++: 1985 to 1989" by Bjarne.  See
  |   discussion there
  |
  ======================================================================*/

extern void * operator new (size_t size, void * p);