More accurate definition of Complex#sign

[HertzDevil]

Fixes #11700. Handles zeroes, infinities, and not-a-numbers correctly.

A minimal definition would be just zero? ? self : phase.cis. The NaN checks ensure we don't run into weird edge cases for the rest of the method's logic. The remaining lines are best illustrated with a code snippet:

# 6.12e-17 is `Math.cos(Math::PI / 2)`
# 1.22e-16 is `Math.sin(Math::PI)`
Complex.new(0, 1).phase.cis                  # => (6.123233995736766e-17 + 1.0i)
Complex.new(-1, 0).phase.cis                 # => (-1.0 + 1.2246467991473532e-16i)
Complex.new(1, Float64::INFINITY).phase.cis  # => (6.123233995736766e-17 + 1.0i)
Complex.new(-Float64::INFINITY, 1).phase.cis # => (-1.0 + 1.2246467991473532e-16i)

# `#sign` ensures these results are exact even when not doing `self / abs`
Complex.new(0, 1).sign                  # => (0.0 + 1.0i)
Complex.new(-1, 0).sign                 # => (-1.0 + 0.0i)
Complex.new(1, Float64::INFINITY).sign  # => (0.0 + 1.0i)
Complex.new(-Float64::INFINITY, 1).sign # => (-1.0 + 0.0i)