for-each-ref: rename some functions and make them public

for-each-ref: rename some functions and make them public

Rename some of the functions and make them publicly available.
This is a preparatory step for moving code from 'for-each-ref'
to 'ref-filter' to make meaningful, targeted services available to
other commands via public APIs.

Functions renamed are:
parse_atom()		-> 	parse_ref_filter_atom()
verify_format()		-> 	verify_ref_format()
get_value()		-> 	get_ref_atom_value()
grab_single_ref()	-> 	ref_filter_handler()
sort_refs()		->	ref_array_sort()
show_ref()		->	show_ref_array_item()
default_sort()		->	ref_default_sorting()
opt_parse_sort()	->	parse_opt_ref_sorting()
cmp_ref_sort()		->	cmp_ref_sorting()

Rename 'struct ref_sort' to 'struct ref_sorting' in this context.

Based-on-patch-by: Jeff King <redacted>
Mentored-by: Christian Couder <redacted>
Mentored-by: Matthieu Moy <redacted>
Signed-off-by: Karthik Nayak <redacted>
Reviewed-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>