More API changes

[chrisbanes]

As we're on a roll of API changes, here's one final change:

• SuccessResult and ErrorResult have now been expanded in the sealed CoilImageState, which now contains: Success, Error, Loading & Empty
• The recent CoilImage with the image composable has been expanded and made more flexible:

CoilImage(
  data = "https://www.image.url",
) { state ->
  when (state) {
    is CoilImageState.Success -> // TODO
    is CoilImageState.Error -> // TODO
    CoilImageState.Loading -> // TODO
    CoilImageState.Empty -> // TODO
  }
}

The more opinionated (and easier to use) version which hosts content slots still exists, allowing:

CoilImage(
  data = "https://www.image.url",
  fadeIn = true,
  loading = { /* content to show while loading */ },
  error = { /* content to show when an error has occurred */ }
)

• The tests have been revamped, using MockWebServer to serve the images in a more realistic way.

Closes #92

[nickbutcher]

s/content/control?

[nickbutcher]

A few small doc suggestions but changes LG. I like the easy default but ability to customize everything if you want.