hydra.el (hydra--head-color): Simplify

hydra.el (hydra--head-color): Simplify

* hydra.el (hydra-face-red):
(hydra-face-blue):
(hydra-face-amaranth):
(hydra-face-pink):
(hydra-face-teal): Improve docstrings.
(hydra--head-color): Simplify.
(defhydra): Use copy-sequence on inherited heads. Move :cmd-name setting
to the very end, when :exit is already set.

* hydra-test.el: Update tests.