Naming of `Token` type

[tesaguri]

While the term "token" in OAuth 1.0 refers to token credentials, oauth_credentials::Token represents a set of client credentials and token credentials, which are not identical concepts. The difference is confusing especially when a user uses the term to refer to the both concepts at the same time, which is the case almost every time they construct a Token.

let token: Credentials = get_token(&client).await; // Token credentials
let token = Token::new(client, token); // Client and token credentials

Also, token.client() and token.token() doesn't sound very well.

So I think Token should be renamed if there is a good alternative. Renaming a type doesn't prevent the semver trick, so it wouldn't cause a breakage provided that it is done before 1.0 release.