Introduce Await component for nicer template usage

[NullVoxPopuli]

Supersedes?: #709

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The existing load function/helper combo gets the job done, but this new component has a similar top-level API but requires no if-based branching. Instead, it uses named blocks to render the resulting state.

import { Async } from 'ember-async-data';

<template>
  <Async @data={{Promise.resolve 123}}>
    <:pending>Still waiting...</:pending>
    <:resolved as |value|>
      The value is: {{value}}.
    </:resolved>
    <:rejected>Whoops! Something went wrong!</:rejected>
  </Async>
</template>

This ends up being both briefer and less error prone, not least since it provides the resolved value automatically from the resolved block, whereas expressly checking {{#if someAsyncData.isResolved}} allowed the further {{someAsyncData.value}} access but did not provide it automatically.

[SergeAstapov]

@NullVoxPopuli do you think we could split infra updates into separate PR to keep this managed/focused?
Also I think .gts conversion for existing tests could be separate PR, e.g. I was doing this as well in #688

[NullVoxPopuli]

@NullVoxPopuli do you think we could split infra updates into separate PR to keep this managed/focused?

Yes, this makes sense -- usually my way of working is to spike all the way to the end / green (so that I know my direction is even valid), and then pull the isolated stuff out.

Also I think .gts conversion for existing tests could be separate PR, e.g. I was doing this as well in #688

ah awesome! I'll rebase once that's merged