bind-keys-form: new keyword :repeat-map, for defining repeat maps

bind-keys-form: new keyword :repeat-map, for defining repeat maps

use-package-normalize/:bind: allow keyword :repeat-map.

bind-keys-form: Add keyword :repeat-map. Specifying a symbol as the
repeat-map defines a keymap with that name (and with the docstring
`repeat-doc', if specified). Symbols for functions bound to keys under
the scope of :repeat-map have their 'repeat-map property set to this
map. Update docstring (and that of `bind-keys') to reflect changes.

Rename `doc' to `prefix-doc' for clarity and consistency with
'repeat-doc'.