Change expressions to avoid unused-type warning.

The warning is new when transitioning from GCC 4.7.2 (MinGW) to 4.8.1
(MinGW-Builds).
Add "constexpr" to default constructors.
Redo conversion constructors for "complex_it."

Create separate same-length, cross-type constructor for "complex_it"
to make those conversions more efficient.  Reduce its non-longer
cross-conversion constructor to take strictly-shorter objects; make
it more efficient since it can now access (shorter) same-length
cross-type conversion without infinite recursion.  Touch up
description for longer-length cross-type constructor.
Redo "complex_it"-to-"complex_rt" constructors.

Change the hidden implementation constructors to take start-indices
as regular function arguments, instead of IntegerConstant types.
(There should be no difference when executed at constexpr time.  And
it saves in instantiations.)  Change the public cross-philosophy
conversion constructors to work with their new implementations.
Redo list-of-shorter-"complex_rt" constructors.

Change the hidden implementation constructors to take start-indices
as regular function arguments, instead of IntegerConstant types, and
to take sub-objects as a single array instead of separate variadic
arguments.  (It saves in number of template instantiations.)  Change
the public sub-barrage (list) conversion constructor to work with
its new implementation.