Update adapter method return types

Using the final types @carlbennettnz and I discussed in #139:

find: Promise<{ primary: Data<Resource>, included?: Resource[], collectionSize?: number }>
create: Promise<{ created: Data<Resource> }>
update: Promise<{ updated: Data<Resource> }>
delete: Promise<{}>
addToRelationship: Promise<{ before?: Relationship, after?: Relationship }>
removeFromRelationship: Promise<{ before?: Relationship, after?: Relationship }>

ethanresnick / json-api

Update adapter method return types #164