RFC: add a dirty flag to Dict

[JeffBezanson]

This implements my suggestion in #9573. Could also be used to detect modification during iteration.

[garrison]

Here's a (minimal) test case to add with this:

# Issue #7944
let d = Dict{Int,Int}()
    get!(d, 0) do
        d[0] = 1
    end
    @test length(d) == 1
end

[JeffBezanson]

Ok, rebased & test added. Will merge when CI finishes.

[garrison]

Awesome, thanks!