proposal: store and maintain element count in oc_list

[martinfouilleul]

Maintaining a count along an oc_list comes up a number of times in the codebase, so it may be useful to add that to the oc_list struct itself. Having that would also allow reverse indexed for loops on list (although this usecase it a lot less frequent)

@rdunnington @Parzival-3141 @Skytrias what do you think? And would it cause problems with the language bindings (since it modifies the length of the list struct and might not map 1-to-1 to language native constructs)

Needed if we agree this is a good idea:

• review places where we maintain a separate count and use list.len instead
• update api.json