Refactor IgnoreChanges

[t0yv0]

The intent here is to refactor the code without any observable changes to make possible further extensions such as #1614 .

Specifically, before this PR:

type Provider interface {
        ...
	Diff(t string, s InstanceState, c ResourceConfig) (InstanceDiff, error)
}

type InstanceDiff interface {
        ...
	IgnoreChanges(ignored map[string]bool)
}

That is the interface is to compute a diff first, and then side-effect to ignore changes.

After this PR we have this instead:

type Provider interface {
        ...
	Diff(t string, s InstanceState, c ResourceConfig, opts DiffOptions) (InstanceDiff, error)
}
type IgnoreChanges = func() map[string]struct{}
type DiffOptions struct {
  IgnoreChanges IgnoreChanges
}

So the algorithm to ignore changes is specified upfront when constructing the diff. This seems to be useful to un-constrain the implementation of shim.Provider a little bit, specifically in #1614 that is trying to change the internal representation of InstanceDiff implementation in such a way that it is inconvenient to process IgnoreChanges after the InstanceDiff is constructed already.

[t0yv0]

Opening for early feedback. We don't have to merge yet I guess until I have an end to end demonstration that 1614 is viable on top of this (I will rebase). But this seems to me like it could help.

[iwahbe]

I left a couple of comments, but I'm over all in favor of a refactor like this.

[t0yv0]

@iwahbe PTAL, I simplified this a bit and removed old way of doing things instead of deprecating it.

[iwahbe]

LGTM