completion/ivy: add ivy-rich; rewrite custom transformer #331

+ivy-buffer-transformer does *most* of what ivy-rich does, so lets cut down on
our own code, bring in ivy-rich, and add our customizations on top of it.

This fixes ivy-use-virtual-buffers support, too.

modules/completion/ivy/packages.el

@@ -7,3 +7,4 @@
 (package! smex)
 (package! swiper)
 (package! ivy-hydra)
+(package! ivy-rich)