Due to issues with preset prefixes in general definers and nested
:prefix's supplied from a map! call not cooperating, many localleader
keybinds were broken and causing errors. For :leader/:localleader keys,
we now use :infix for sub-prefixes.
However, with this change, the :alt-prefix property has been removed, as
there is no simple way to support this without some major state
gymnastics in map!.
Fixes#1059