options for complex numbers construction

[moodymudskipper]

Let's postpone this for after v1, but it would be nice to be able to :

• construct using only the complex() function -> a "complex" constructor
• be able to call it once or for every item -> c(complex(...), complex(...)) vs complex(real = c(...), imaginary = c(...), recycling if single value on either side. That's a new option, name TBD.
• be able to provide polar coordinates -> arg polar = FALSE, that applies only for "complex" constructor.