helper/schema: track map element counts

This adds "field.#" values to the state/diff with the element count of a
map. This fixes a major issue around not knowing when child elements are
computed when doing variable access of a computed map.

Example, if you have a schema like this:

    "foo": &Schema{
        Type:     TypeMap,
        Computed: true,
    }

And you access it like this in a resource:

    ${type.name.foo.computed-field}

Then Terraform will error that "field foo could not be found on resource
type.name". By adding that "foo.#" is computed, Terraform core will pick
up that it WILL exist, so its okay.

@svanharmelen Since you've been in helper/schema recently, I'd appreciate a review. Thanks!