refactor resumable

[acjay]

(this is ugly because I just grabbed this TODO from notes I left in the comments as I was working)

• give resumable variadic args when resumable is first called, like makePipeline. Then get rid of initalArg parameter
• ^, then pass initial args to all functions in resumable, like makePipeline
• ^ and collect intermediate results of reusumable in an array, which is always the first result
• ^ and return partial results in error object
• make context an optional argument of resumable -- then it can be set either that way or by using call/apply