[size: tiny] Rename methods exposed by ReactScheduler

[flarnie]

Going to make incremental PRs with the changes we discussed. This is a quick and easy one to start with.

what is the change?:
rIC -> scheduleSerialCallback
We will later expose a second method called scheduleDeferredCallback,
for different priority levels.

cIC -> cancelScheduledCallback
This method can be used to cancel callbacks scheduled at either serial
or deferred priority.

why make this change?:
Originally this module contained a polyfill for requestIdleCallback
and cancelIdleCallback but we are changing the behavior so it's no
longer just a polyfill. The new names are more semantic and distinguish
this from the original polyfill functionality.

test plan:
Ran the tests

[bvaughn]

if rIC -> scheduleSerialCallback, then shouldn't cIC -> cancelSerialCallback ?

[gaearon]

I don’t think so.

We will later expose a second method called scheduleDeferredCallback,
for different priority levels.

This method can be used to cancel callbacks scheduled at either serial
or deferred priority.

(Emphasis mine)

[bvaughn]

Ah, my bad. I skimmed the PR description and missed that.

Seems a little odd, initially, to have separate scheduling methods and a shared cancel method, but I haven't thought about it much. 😄

[flarnie]

Yea, I had initially assumed we would have a separate cancel method for each priority, but we don't actually need different behavior for cancelling. It's always the same.

[acdlite]

Let's bikeshed these offiline. I don't like scheduleSerialCallback because it's the event that's serial, not the callback itself. And I don't like scheduleDeferredCallback because part of the reason for creating this module is to blunt the negative connotation of words like "async" and "idle" and "deferred." We've observed product engineers are scared to wrap anything in these APIs, even stuff like network events, because they connote slowness.

[flarnie]

@sebmarkbage had brought up that we haven't renamed these methods yet - so let's just bookmark that we're holding off on choosing names until we have time to discuss at length.