Adapt to the new `hydra-set-transient-map'

Adapt to the new `hydra-set-transient-map'

* hydra.el (hydra-set-transient-map): Amend arglist.
(hydra--clearfun): Don't count on there being another transient map.
(hydra-disable): Update.
(internal-push-keymap): Define unless defined.
(internal-pop-keymap): Define unless defined.
(hydra--pred): Remove.
(hydra--universal-argument): Update.
(hydra-last): Remove.
(hydra--aggregate-color): Remove.
(hydra--unalias-var): Remove.
(hydra-pink-fallback): Remove.
(hydra--modify-keymap): Remove.
(hydra--make-defun): Update.

Re #90