Add Resolvable types

Several methods could benefit from abstract resolvable types for parameters.

For example: `Groups#add(user)` could take a user ID string, a `User` object, or even a `Member` object.

Some possible resolvable types:
```ts
type UserResolvable = string | User;
type MemberResolvable = string | Member; // could User work here too?
type GroupResolvable = string | Group; // would it be a good idea to include Member here?
```

The corresponding class would have (static?) methods for resolving these.
```ts
Group.resolve(GroupResolvable): Group // do these need to be async...?
Member.resolve(MemberResolvable): Member
User.resolve(UserResolvable): User
```