Add padding flag.

Add class-static constexpr Boolean flag "has_padding" to "complex_it"
and "complex_rt" (both cases).  The flag is TRUE only when the type is
bigger than the combined size of the stored components.  When FALSE, a
packed run of complex numbers can be reinterpreted as a packed run of
components.
Add "norm" function.

Add the (Cayley) "norm" function, which computes the squared-magnitude
of a hypercomplex number.  (It's also the product of a number with its
conjugate.)
Add "real," "imag," and "unreal" free-functions.

Add free-function versions of "real," "imag," and "unreal" to both
"complex_it" and "complex_rt."

Remove ADL assistance in some old unit tests.  Add double-negative check
to "conj" unit tests.
Rearrange operator definitions.
Add addition and subtraction operators.

Add operators + (binary), - (binary), +=, -=, --, and ++ to "complex_it"
and "complex_rt" (both cases).  Some combinations of operands for the
binary operators include a scalar (of value_type).

Add missing comment header to two unit tests.

Include descriptions of complex-conjugation being an (anti-)involution.