Context abstractions around API and the state cache

[diamondburned]

Allow copying the API struct with a non-nil context, which would allow people to inject a Context in.

Example:

ctx, cancel := context.WithTimeout(context.Background(), time.Second)
defer cancel()

var err error

api := session.API.WithContext(ctx)
err = api.Guild(123123) // 500ms each request
err = api.Guild(123123)
err = api.Guild(123123)
err = api.Guild(123123) // cumulative: 2 seconds

if err != nil {
	log.Fatalln("Failed to fetch guild:", err) // context expires, errors
}

Since the state also has an API abstraction, it could also have a similar method:

ctx, cancel := context.WithTimeout(context.Background(), time.Second)
defer cancel()

cstate := state.WithContext(ctx)
cstate.Guild() // will use context with copied API instance.