Simplify toke usage stats api

[fogfish]

from

type Chatter interface {
	UsedInputTokens() int
	UsedReplyTokens() int
	Prompt(context.Context, []fmt.Stringer, ...Opt) (*Reply, error)
}

to

type Chatter interface {
	Usage() Usage
	Prompt(context.Context, []fmt.Stringer, ...Opt) (*Reply, error)
}