standardize attribute, method, argument, etc names

[tybug]

every forward-facing name needs a pass to make sure they're consistent and make sense, as the names given by the api do not always do either. For example:

• Search.{user, wiki_page} should probably be Search.{users, wiki_pages}
• we might want to rename User.id to User.user_id, and similarly for Beatmap and Beatmapset (I'm not entirely sold on this, but a straight id attribute isn't very intuitive to me. Perhaps others feel differently)

[tybug]

a prerequisite to this is a custom field class which we can use to tell our deserialization to deserialize an attribute into a differently named field, since right now they have to match 1:1, which means we can't rename user to users or wiki_page to wiki_pages. Such a field class would also allow us to get rid of our special-case handling for the @ character in attribute names, which we currently force-replace with an underscore.

[tybug]

closed by 7a77f5b. I've since decided that id instead of user_id or something similar is actually probably fine. I think I slightly prefer user_id but not enough that I can justify breaking parity with the api.